DIY crucible for melting metal (for lead, aluminum, silver, etc.)

DIY crucible

To melt metals, special heat-resistant bowls called crucibles are used. They are very popular in jewelry workshops, laboratories and the metallurgical industry. But for a full-fledged process, it is not enough to acquire a simple object with a heat-resistant surface, because different iron requires its own product, which must correspond to the chemical composition and be suitable for a specific temperature regime. These facilities also produce a finished alloy, which remains to be given the correct shape.

Sometimes it happens that such devices may be needed in private business, but purchasing them is expensive. Therefore, it is more profitable to make a crucible with your own hands - you can save a significant share of the budget. Yes, the procedure will require certain skills and patience, but in the end you will get a vessel that is not inferior to factory analogues. It is also important to decide on the types of objects to be melted in order to make a suitable bowl. If you plan to work with different metals, it is recommended to create several products.

Aluminum melting crucible

Aluminum can be processed using graphite crucibles, which are capable of providing melting temperatures in the range from 400 to 1400 °C. In particular, so-called mammut crucibles are widely used. The widest temperature range for aluminum melting is offered by universal containers. They provide both intense heating and direct melting. In the middle temperature range, an aluminum crucible is used, immersed in an electric furnace. Such forms, in addition to temperature resistance, are distinguished by protective coatings against corrosion and mechanical durability. The most stable crucibles for servicing aluminum are used in gas furnaces, which provide a temperature regime of 1400 °C. Less commonly practiced is the use of a combination of crucibles with liquid fuel furnaces.

General manufacturing steps

To begin with, the raw materials are prepared, and here everything depends on the model of the future tank. It is better to take components with a reserve, because the first thing is unlikely to work out. Also, for safety reasons, you should carry out production away from open fire, and choose a well-ventilated area.

It is safer to carry out work in a garage or special extension.

The second stage is mixing the materials and giving the casting the necessary parameters. For these purposes, plaster molds are used. Creating the outlines is not difficult, and such information is easy to find on the Internet. Then a homogeneous material is pasted over the outer part of the model, forming a future homemade refractory crucible. It is also important to give it the required depth and thickness.

Clay crucible in the drying process

And the last step is the drying process: the workpiece is placed in a cardboard box and covered with a lid. This will allow the casting to dry and remove excess water from it. Sometimes heat treatment may be required, however, the important point is to control the annealing temperature and protect the skin of the hands and face. If the heat is too intense, the item will burst and there is a chance of severe burns. Detailed instructions on how to make a crucible yourself and at home will be described in the following chapters.

Making a clay crucible

Here you can’t do without fireclay clay, which is sold in any building materials store. It tolerates extreme thermal effects very well, is cheap and there are unlikely to be any problems finding it. As a last resort, you can make a crucible from crushed fireclay bricks. You will also have to buy liquid glass and mix all the ingredients for a homogeneous base. The proportions look something like this:

All components are added in stages: clay and fireclay are mixed until smooth, and water is gradually added to them. The main goal is to create a mixture that will not stick to your hands. When the required consistency is obtained, glass is added and everything is thoroughly mixed. The main thing here is to bring the object to a state where the plane stops cracking. The mixture is ready, and for storage it is recommended to use thick cellophane, or wrap it in 7-10 layers of film.

Before sculpting, you will have to remove any remaining air by hitting the substance about 8-12 times on a hard surface.

The mixed material is applied inside the model, its depth and thickness are formed. It is better to create a semicircular bottom, which will give a greater effect during future melting of iron filings. Also, the substance must be pressed tightly against the model so that air does not form between the planes, and for greater convenience it is recommended to wet your hands with water.

Afterwards, the tank is sent for drying: it is placed in a container made of cardboard or plastic and placed in a dry place. A few hours will be enough to remove any remaining moisture. Also, the product will settle a little, and it will be easy to remove it from the mold. A refractory vessel made of fireclay bricks will be enough for a long time of use, however, the last point of creation should be the firing procedure in a furnace and at T = 800 °C. And the thing can be used for its intended purpose. For ease of use, you will need a crucible furnace, which you can make yourself. For simple installation, you can weld a structure from several pipes to form a cylinder. Usually it is fixed on two parallel posts so that it does not touch the ground. And here the thickness of the walls (minimum 5 mm) and the stability of the product are taken into account (it must easily withstand T = 1600 °C or more).

Brass grades and applications

The brand of brass and its scope of application depend on the composition. For example, tombak, which belongs to the class of wrought brasses, which contains more than 95% copper, can easily combine with steel, forming a bimetal with it. This compound is used in the manufacture of insignia and various objects of art and interior design - figurines, frames, candlesticks .

LO grade brass is used for the manufacture of condenser tubes used in various heating equipment, for example, gas boilers, autoclaves, and bellows.

The LS brand is used to create parts for watch mechanisms, adapters and connecting bushings. Printing matrices are also made from it.

LMC - is found in old Soviet coins with denominations up to 5 kopecks, fittings, nuts and bolts, and its subtype with the prefix “A” is found in parts of river and sea vessels.

Brass, marked LA or LZhM (and its subtypes), is also used for the construction of sea ships and aircraft, various electrical machines and bearings. Very common in parts for various chemical equipment.

How to make a graphite crucible

Capacity of this category has many advantages:

If you take the easy route, you can take a graphite rod, and the crucible is almost ready. All that remains is to attach the bottom.

Graphite crucibles in various sizes

If the required tube is not found, everything can be done using two molds of different sizes, which are inserted one into the other, and the free space will allow you to give the desired dimensions. Initially, you need to pour the mortar into an empty container, and you should not spare it. The fact is that the powder will compact and settle. Next, add liquid glass (about 15 ml) and mix everything thoroughly. It is recommended to place the mixed mass in a large cylindrical container (you can use a plastic cup) and press a hole into a small one, leaving the bottom thick enough.

As a result, a vessel will come out, which is given time to dry. In this case, heat treatment will also be required to remove excess liquid. If all the steps were correct, then you will have a high-quality graphite crucible, made by yourself.

Lead crucible

Crucibles of almost all common types can be used in molten lead, with the exception of fragile structures. For example, ceramic, porcelain and clay models are not recommended for use in this case. Of course, the same graphite crucible may be optimal, but its high cost limits the possibilities for application. It should be noted that the crucible for melting lead is not so common in the professional sphere as in private households. Metal is used to make shot, weights, various molds and other devices that may be needed on the farm. The optimal solution for such melting of a house may be a combined composition of graphite with affordable construction additives.

Assembling a cast iron crucible

This type is the worst, but sometimes it brings good benefits. All that is required is to place a cast iron glass of a smaller diameter in a metal bowl, and fill the free space with sand and clay.

DIY cast iron crucible

Next, everything is heated in the oven until the mixture melts and takes on the same type of substance. Afterwards, the cup will harden, and iron can be melted in it. This is basic information on how to make a crucible at home and at minimal cost.

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Why use graphite?

• The crucible furnace can use containers made of pure graphite or a mixture of graphite powder with other binding materials. The former are more resistant to fire and stronger, having the ability to surround the metal melt with a reducing atmosphere. Crucibles made of graphite powder and additional components are endowed with less wear resistance and become dirty more quickly.
• Almost any metals and alloys can be melted in graphite crucibles.
• They have a long service life and can withstand even temperatures up to 2500˚C, which can significantly speed up the work process. Moreover, high temperatures in a crucible furnace only increase the strength of such containers.
• Graphite crucibles can be reused many times, the main thing is to remember to clean and polish them repeatedly.
• Their electrical conductivity allows the melting of precious metals without any problems.

DIY fireclay crucibles

Making crucibles from fireclay clay

Porcelain crucibles, even at not too high temperatures, often crack and burst. It is not economically profitable to use crucibles made of other more expensive materials (graphite, alundum, etc.). However, the prices for laboratory porcelain are also not encouraging.

The photo shows a broken porcelain crucible.

For the work, a porcelain crucible (as a model), alabaster (construction plaster) and a plastic bottle of mineral water were used. The bottle is cut with the following calculation: crucible height plus 40-50 mm. This will be the form. Alabaster is poured into it and water is added. The consistency should be like liquid sour cream. A porcelain crucible is sunk into the mixture (approximately in the center) to the very edge. You need to put something heavy in the crucible, so that it does not sink and “float up”. After the alabaster has hardened, you need to remove the crucible from the plaster and the mold from the scrap of a plastic bottle. The plastic is cut with scissors and easily tears along the cut line, but removing the crucible from alabaster is somewhat more difficult.

The photo shows how to do this:

The plaster mold is placed on two boards in such a way that the porcelain crucible is between them. Then, with a strong blow with a wooden board (mallet, etc.), the crucible is knocked out of the mold. To ensure that the mold does not collapse, its bottom is specially made with a thickness of 40-50 mm.

The form is almost ready. If there are defects on its inner surface - cracks, bubbles, etc. they are covered with liquid alabaster. After drying, all irregularities are smoothed out. There are several ways to make a clay crucible according to your mold. One of them is the so-called. slip casting. This method has both pros and cons. It is more rational at the initial stage to simply sculpt the crucible in a mold, rather than fiddling with liquid clay.

Before sculpting, the prepared clay is “beaten” to remove air from it. To do this, you need to beat the lump of clay onto a hard surface (cover the floor with newspaper and forcefully throw the lump of clay onto it). And so five to seven times. When the clay is ready, take a small lump and forcefully press it to the bottom of the mold. Then small pieces of clay are taken and the walls of the crucible are formed from them. The wall thickness is controlled along the edge of the mold. The most important thing is to carefully knead the clay into the mold. You can smooth out unevenness and make the inside of the future crucible smooth by moistening the clay with water. But this is the last thing. The next very important stage of work is drying. The form is placed in a cardboard box, which is covered with a lid. As the water evaporates, the clay shrinks. After 6-8 hours, the crucible will decrease in volume, and removing it from the mold will not be difficult - just turn it upside down and it will fall out:

After this, the crucible is dried in a cardboard box without a mold. As it dries, all defects that appear are eliminated. If cracks have formed, they are covered with liquid clay.

After the crucibles have dried, as in the photo, seven crucibles are gray, two are darker - they are not yet dry. The first seven can already be burned. To do this, it is advisable to use a muffle furnace:

The firing temperature is eight hundred degrees. Enough for the crucible. When firing, it should be taken into account that the clay contains organic matter that will burn out and water in a bound state that will evaporate. Accordingly, you need to worry about the release of gases from the oven. A laboratory muffle furnace has a hole in the door for this purpose. If it is not possible to use such a kiln, you can fire one at a time in a small one:

Preforms can also be used to make crucibles.

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Clay crucibles

The popularity of clay crucibles is due not so much to their positive performance qualities as to their availability and ease of manufacture. Typically, for such needs, fireclay clay is used, which during the manufacturing process is subjected to three stages of processing - creating a working mass, modeling and drying. This technology has replaced porcelain containers. They could also be molded without much effort, but in practice they did not meet the requirements for strength and resistance to high temperatures. As a result, cracks and chips appeared. What are clay crucibles in practice? This is a stronger material than porcelain, even at elevated temperatures. However, in terms of other qualities of the same interaction with metal alloys, clay is not so harmless - at least compared to alundum and graphite products. Nevertheless, economically, the use of clay crucibles more than justifies itself when it comes to small typical melting operations.

How to make a crucible or melting furnace with your own hands

Almost every item has several types and purposes, including stoves. There are stoves for heating rooms and for cooking food, and there are special devices for melting metals or storing them in molten form. Such devices are called crucible melting furnaces. They have a specific purpose and therefore the list of enterprises where they have found their application is quite small. These are mainly factories and laboratories. But what to do if you need to melt metal for some purpose at home? Buying such equipment is very expensive, but it is quite possible to make it yourself. This requires minimal knowledge in this area, desire and time.

Functions of crucibles

As already noted, the crucible is used in metal melting processes as a load-bearing container. However, even within this simple function, the specific tasks of such devices may differ. The most common use of a crucible is as a separate container for organizing metal melting. In this way, in particular, the metal charge in metallurgy is serviced. But there are other ways of application, including as part of induction equipment. It would seem, why do you need a crucible in an induction furnace if it can be heated autonomously? There are many reasons for this. Firstly, to ensure the purity of the alloy itself and the supply of high temperatures. The crucible acts as a kind of utensil that protects the metal from impurities and unwanted gases. Secondly, the surfaces of the internal walls of the induction furnace are also protected. There is a third way to use crucibles. With the help of special mechanisms, metallurgists can move metal masses in a molten state using similar containers.

Types of crucibles

A crucible furnace is a container made of refractory material in which metal is heated to a certain temperature.
The main materials from which crucibles are made: Crucible furnaces are used both in factories where metal products are made, and in small enterprises, for example, for the manufacture of jewelry.

Ceramic stoves are the best option. When metals are melted in a ceramic crucible, no changes occur in the substance itself. even base metals or alloys of cobalt, chromium or palladium in such crucibles without any problems

Graphite crucibles. Such furnaces are characterized by a long service life and high resistance to oxidation, which makes them universal for melting any metals and especially alloys based on zinc and brass. In addition, they are often used in induction furnaces. Graphite crucibles can withstand very high temperatures, such as eight hundred degrees to melt aluminum.

Cast iron crucibles are perhaps the worst of the three listed. They have high reactivity, rapid oxidation and interaction with other metals, and cast iron does not resist high temperatures well. For these reasons cast iron crucibles are very rare , but they are inexpensive and quite accessible.

This article will look at methods for making three types of homemade crucibles.

Material selection

Here, fire-resistant components play a serious role, which include:

1. Ceramics is an average option, perfect for personal use. There are no reactions that can change the structure of the metal in this cookware, and it is excellent for cobalt, chromium and palladium.
2. Clay is a substance that is used in the production of crucibles for jewelers. This component is highly fire-resistant and can withstand up to +1600 °C. If a person wants to create jewelry in his own room, but does not know what to use for melting, then this option is definitely the best.
3. Graphite is excellent for melting galvanized and brass alloys, and its main advantage is durability. As for the operating temperature, it should not exceed +800 °C.
4. Cast iron. Crucibles made from this substance are rare and belong to the budget categories. Also, products of this kind will have disadvantages in the form of rapid oxidation, low heat resistance and fast production (up to 30 heats).

Varieties of homemade crucibles

As an alternative, you can use an electric crucible, which you can do yourself without much difficulty. It has several uses, but the main one is gold refining.

Inductor assembly

The heating element of the crucible arm at home is usually an inductor. It has a cylindrical shape with a cavity inside. A homemade crucible with metal shavings is placed in this cavity. The inductor is made of fire-resistant material , inside it there is a wire winding, most often copper wire is used.
Using a special generator, current is supplied to this winding, which creates an electromagnetic field. Which, in turn, creates an eddy current in the crucible and in the metal placed in it. They melt the chips. The inductor itself is assembled from 4 vacuum tubes with a parallel connection. Such an inductor can be connected to a regular outlet. There is another option for assembling an inductor with your own hands from an electromagnetic core and two layers of winding. The first layer is 10 turns of copper wire with a thickness of 4 mm, and the second is one turn, the material for which is a metal plate with a cross-section of 15 * 5 millimeters. The electromagnetic core is U-shaped and consists of a set of steel plates. The first winding is made around the plates, which is placed in an insulated housing; the secondary winding connects the core and metal bars , between which there should be a distance equal to the dimensions of the crucible. This entire structure is placed in the furnace body.

So, we get a furnace in which the inductor is located. Wires go from the inductor to the socket. A crucible is placed in this furnace in such a way as to enclose the bars. If it is placed correctly, a buzzing sound will be heard, indicating that tension has appeared and melting has begun. If there is no sound, then use the handle to move the crucible until the circuit is completely closed.

About labeling

The most common laboratory crucibles are cylindrical or conical in shape. Melting boats and cups remain easy to use and in demand. In any case, such devices are assigned a number that determines the volume of capacity.

Markings in the range of 1-300 remain generally accepted. If we consider the metallurgical sphere, a conventional unit in it corresponds to 0.142 dm3, but provided that a crucible made of a suitable material is filled by 85% with the metal being processed. Foundry containers differ in the following parameters;

• color;
• size;
• configuration.

If we talk about porcelain crucibles, their parameters are less than 10-15 mm.

Making a crucible from clay

You can make a crucible from fireclay clay. This is an inexpensive option and also highly resistant to high temperatures. This clay is used for laying stoves and can be purchased at any hardware store.
Fireclay clay can withstand temperatures up to 1600 degrees Celsius. So, you will need fireclay clay (sold in bags in hardware stores), liquid glass (sold there) and ground fireclay. It can be bought or made from fireclay bricks.

In order to make a mixture from which a crucible will be fashioned in the future, take 7 parts of clay, 3 parts of fireclay and 10 tablespoons of liquid glass per liter of dry mixture. Fireclay and clay are mixed until smooth. After this, water is slowly added. In order not to spoil the workpiece, you can pour out part of the mixture , and in case of a large amount of water, add dry powder. You need to knead until the clay stops sticking to your hands.

Only after the clay of the desired consistency has been mixed can glass be added. When adding glass, you need to thoroughly knead everything until the clay stops cracking. It is best to add glass to a lump of clay and roll it into a roll, then fold it several times and repeat the procedure until it stops cracking. The material for the crucible is ready. Until the moment when it is used, it must be stored in several layers of cellophane.

There is clay, now to make a crucible you need to take a mold, the easiest way is to use a plaster mold. How to make such a form can be found on any website on plaster modeling. So, directly making the crucible.

Before you start sculpting, you need to knock all the air out of the clay; to do this, you can lay a newspaper on the floor and forcefully throw a lump on it several times, ten times will be enough. Now take a lump of clay and carefully press it into the bottom of the mold, after which the walls of the product are formed in small lumps. Their thickness can be controlled along the edge of the mold. very important to carefully press the clay into the mold so that no air cushions form there. After the crucible is sculpted, you need to make the inner surface smooth. To do this, just moisten the clay with water.

After this comes the moment of drying. The mold with clay is placed in a cardboard box and covered with a lid. After seven hours, all the water from the clay will evaporate and the shape of the future crucible will “shrink” a little, so getting it out of the mold is not particularly difficult. After this, the crucible continues to dry in the same box; as it dries, all defects will be eliminated by themselves and the pot will acquire a gray color. Sometimes small cracks may appear. They can be covered with wet clay. Next, the pots are fired at a temperature of 800 degrees in a muffle furnace. After firing, the crucible is ready for use.

How to make a mixture for a neutral type ceramic crucible?

First you need to prepare the form of a homemade crucible for melting lead.
This is done according to the following algorithm: Dry clay is mixed with ground fireclay until a homogeneous consistency is obtained. It is recommended to make 20 revolutions in the fireclay mill.

• As soon as the required state is achieved, you can unload the mass and begin mixing by hand, adding water little by little. The mixture, in the right condition, will form a lump and will not stick to the skin or leak through your fingers.
• Now you need to add silicate stationery glue.
• Mix the whole mass thoroughly. It is worth noting that this is the most difficult and time-consuming stage. Continue processing until a homogeneous consistency is obtained.
• The next step in creating a crucible for melting lead with your own hands will be to remove air from the finished mixture. It is necessary to ensure that not a single bubble remains in it. Otherwise, the crucible will burst when overheated. To avoid this, the air must be knocked out.
• Spread a film on the hard floor (never use newspapers!).

• Now the fun begins. To eliminate all the air from the workpiece, it is necessary to throw it onto the film with maximum force. Do this until the bubbles stop coming out of the mass. After that, throw 10 more times (more is possible).
• After the procedure for removing air from the mixture has been completed, it can be stored. To keep it for as long as possible, you should use glass containers with a hermetically sealed lid.

Attention! Do not store the workpiece in a plastic container or wrapped in several layers of film. It will dry out in a couple of weeks and will be unusable!

Graphite crucible

Graphite is a material that has many unique properties. Positive qualities of graphite:

To make a crucible from this material you will need:

Some of these materials can be used as independent units. For example, a graphite tube is essentially already a crucible; you just need to make a bottom in it.

The principle of manufacturing from all materials is the same. Let's look at the example of mortar. Two forms are made. You can roll it out of thick paper to make it easier to remove later. The outer shape has the configuration of a hollow cylinder , while the inner one is simply a cylinder. The small cylinder is inserted into the wider one. The mixture will be poured between them. The mold is placed in a plastic cup and mortar powder is poured into it. You need to fall asleep with a slide, as it will sit down when you need to compact it. 15 cubes of liquid glass are poured into this powder using a syringe. Everything is mixed and the consistency of shortcrust pastry is obtained. Stuff into the mold in small portions.

The result is something like a glass turned upside down. To prevent the form from sticking to the table, it is best to do the entire procedure on cellophane. Then the mold is turned upside down and the inner cylinder is removed. It is also best to initially glue it with cellophane or tape. Then, when removing, the shape of the crucible will not be damaged.

After the crucible has dried, it must be placed in the inductor and heated. This must be done at low temperatures , since all the water should evaporate, despite the fact that outwardly it seems as if it is not there at all. If the crucible is not preheated and you immediately start melting in it, it will most likely burst. After warming up, when you tap the crucible, it will make a ringing sound. This indicates that the crucible is well made.

By following the instructions presented, you can quite easily acquire a homemade melting furnace that will last no less than a purchased one. The main thing is to take your time, be careful in your work and not violate manufacturing technologies.

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Step-by-step instructions for melting copper

Copper smelting, if you prepare everything necessary for the implementation of such a technological process and approach its implementation correctly, allows you to produce copper products for both decorative and purely practical purposes even at home.

To melt copper, you will need the following tools, equipment and supplies:

• muffle furnace (preferably with adjustable heating temperature);
• a crucible in which you will melt copper (for melting copper, crucibles made of ceramics or refractory clay are used);
• tongs with which the hot crucible will be removed from the furnace;
• hook (it can be made from ordinary steel wire);
• household vacuum cleaner;
• charcoal;
• the mold into which the casting will be performed;
• gas burner and forge.

Electrical copper contains the least amount of impurities

The crushed copper is placed in a crucible. Keep in mind: the smaller the pieces of metal, the faster it will melt. After filling the crucible with copper, it is placed in a furnace, which, using a temperature regulator, must be heated to the required state. The doors of serial muffle furnaces must have a window through which you can observe the melting process.

The viewing window will allow you to control the process without opening the door again, thereby not reducing the temperature in the oven

After all the copper in the crucible has melted, it must be removed from the furnace using special tongs. There is always an oxide film on the surface of the molten copper, which must be moved to one of the walls of the crucible using a steel hook. After freeing its surface from the oxide film, the molten metal should be poured as quickly and carefully as possible into a pre-prepared mold. The details and rules for performing this procedure are well demonstrated in a video that is easy to find on the Internet.

You will have to pour the metal into molds very quickly if the heating method you chose could not provide the desired temperature

If you do not have a muffle furnace at your disposal, you can heat the crucible with copper using a gas burner, placing it vertically under the bottom of the container

In this case, it is important to ensure that the flame of the gas burner is evenly distributed over the entire area of ​​​​the bottom of the crucible

If at home you need to melt low-melting alloys based on copper (brass and some brands of bronze), then you can use a regular blowtorch as a heating device, also placing it vertically under the bottom of the crucible. When melting is performed using this and the previous methods, the surface of the molten metal will actively interact with oxygen, which will lead to intense oxidation. To reduce the intensity of oxidation, molten copper can be sprinkled with crushed charcoal.

Melting copper with a blowtorch in a homemade stove

If you have a forge in your home workshop, it can also be used to melt copper, brass or bronze. In this case, a crucible with crushed metal is placed on a layer of hot charcoal. To make the heating and melting process more intense, air can be supplied to the coal combustion zone, for which a regular vacuum cleaner that works not to draw in, but to blow out is suitable. If you use a vacuum cleaner, you need to make a metal tip with a small diameter blowing hole on its hose.

The smelting process will be even more efficient in a gas furnace

When selecting a muffle furnace for performing foundry operations with copper and its alloys, you should pay attention to the temperature conditions that such a device can provide. Depending on the type of metal being melted, such a furnace must provide the following heating temperatures:

• copper – 1083°;
• various grades of bronze – 930–1140°;
• brass – 880–950°.

It is possible that you will decide to make a smelting furnace yourself after watching the video.

Ordinary copper, which does not contain any alloying additives in its chemical composition, does not have good fluidity in the molten state, therefore it is not suitable for the manufacture of casting products of complex configurations and small sizes. For these purposes, it is best to use brass, and choose an alloy whose surface color is lighter (this indicates that brass of this brand has a lower melting point).

Crucible: purpose, principles of melting, manufacturing, options, diagrams

Author: Yuriy Fedorovich Kolesnikov, thermal power engineer*

© When using site materials (quotes, images), the source must be indicated.

Crucible is a vessel for melting metal. As a rule, conversion metal is melted in crucibles, i.e. already brought to the required degree of quality for casting into a mold or refining (deep purification from impurities). The general line of development of large-scale metallurgy is to reduce the number of processing steps, up to the release of conditioned metal directly from the melting furnace, but in industry crucible melting still retains significant importance, and in handicrafts and jewelry it dominates.

The crucible is not just a fairly heat-resistant vessel. Its chemical composition and design must correspond to the type of metal being melted and the melting mode. This article describes how to make a crucible with your own hands and what conditions it must satisfy for use at home or in a small workshop. For beginner metallurgists, you will first have to touch on the metal smelting process itself, because... The requirements for the crucible are determined mainly by its conditions.

Melting metal in a crucible at home

Features of graphite crucibles

Products made of graphite and carbon demonstrate increased chemical and thermal resistance, which determines their value in metallurgy. For example, special graphite compositions can withstand temperatures of about 3000 °C. It is obvious that such a crucible can be used for melting metal in almost all areas of the metallurgical industry. The equipment is used for melting, distributing, pouring non-ferrous and ferrous metals - including in combination with induction vacuum furnaces. The manufacturing technology of graphite crucibles makes it possible to precisely calculate their shape and dimensions down to the smallest detail. Most often, the dimensions are oriented to the specific design of the induction furnace. For example, forms in the form of glasses are common, where other technological additions are also present - spouts, collars, holes, etc. But graphite raw materials are much more expensive than other materials, so the equipment is used mainly in the professional field. Although there are exceptions when home craftsmen dilute the compositions with cheaper materials.

A little about melting

In a deep vacuum, the high-purity metal being melted can be heated exactly to the melting temperature or slightly higher, and kept at it for some time so that tiny, literally a few atoms, remains of crystallites melt. Then the metal can be allowed to cool slightly below its melting point - it will remain liquid, like a supersaturated solution without a seed crystal. If we now pour the metal, also in a vacuum, into a mold made of a chemically absolutely inert material, in which a seed crystal of the same metal is placed, then, observing all the subtleties of this technology, we will obtain a single-crystal casting with unique properties.

In amateur conditions, vacuum melting, alas, is not feasible. In order to properly make a crucible for melting metal yourself, you need to take into account a number of features of melting in a non-inert chemical gas environment. The melted metal, firstly, interacts with air, causing part of it to be lost to the formation of oxide, which is especially important when melting scrap precious metals: at its melting temperature (1060 degrees Celsius), even gold noticeably oxidizes. To compensate to some extent for oxidation, the crucible must provide a reducing environment for the melt or be chemically inert if the metal is melted with a clean open flame, see below.

Secondly, so that the metal in the crucible does not freeze until it is brought to the casting mold, so that the remnants of the original crystallites do not spoil the casting, and the melt acquires sufficient fluidity, the metal in the crucible is overheated. For example, the melting point of zinc is 440 degrees, and its foundry temperature is 600. Aluminum, respectively, 660 and 800. Since overheating of the metal after melting takes some time, degassing of the melt also occurs at the same time, this is the third thing.

Recovery

In metallurgy, atomic carbon C, carbon monoxide CO (carbon monoxide) and hydrogen H are used as reducing agents. The latter is most often an accidental guest, because for this purpose it is too active and is absorbed by metals without forming chemical compounds with them in large quantities, which spoils the casting material. For example, solid platinum at room temperature can absorb up to 800 volumes of hydrogen. A platinum blank in a hydrogen atmosphere literally swells before our eyes, cracks and falls into pieces. If you take them out of the hydrogen chamber and heat them, hydrogen will be released back.

Note: in a similar way, but in smaller quantities, metals absorb/emit other gases, e.g. nitrogen. This is why degassing of the melt is required, see also below.

A noticeable proportion of hydrogen reduction occurs when heated by an open flame of a gas burner, upon its contact with a less heated surface. The metal does not deteriorate - the absorbed hydrogen is released and burned later in the smelting process. But, if the crucible material is also prone to gas absorption, it may crack and burst during melting; this must be kept in mind.

CO reduction is noticeable if the metal in the crucible is melted by the open flame of a liquid (gasoline, kerosene, diesel) burner, for the same reasons. Liquid fuel burns much slower than gas, and its afterburning zone extends several cm from the burner nozzle. Reduction with carbon monoxide is the cleanest from the point of view of the metal: it does not spoil the metal and does not produce by-products with a strong excess of the reducing agent. Therefore, CO reduction is widely used in metallurgy when smelting metal from ore, but no one has yet figured out how to make a crucible furnace (see below), in which oxidation compensation would be completely provided by CO.

Atomic carbon is a reducing agent energetic enough to compensate for oxidation. It is also not difficult to create a reducing environment in a crucible using C: it is enough to introduce free carbon in one or another allotropic modification into the composition of its material or make the entire crucible from a heat-resistant and mechanically sufficiently strong allotrope C; graphite is one of them. When reducing C, there is a danger of carburization of the melt, but graphite releases very little atomic carbon when heated. If you heat the metal in a graphite crucible with a gas flame, then the excess C will immediately find a more “tasty” H for it and the danger of carburization will be reduced to zero. And for other heating methods (see below), you can select the dimensions, configuration of the crucible and the addition of graphite to its material so that there is simply no excess C under any conceivable melting mode. This is a very valuable property of graphite, keep it in mind too.

Note: the coefficient of thermal expansion of TKR graphite is negative, which significantly compensates for the thermal expansion of the crucible, increases its durability and increases its service life. Also valuable quality.

Excerpt

So, it’s clear why the melt in the crucible needs to be overheated and held. Although metal casting is a completely different topic, it still needs to be mentioned here that the melt holding time should be observed quite accurately. Chemically pure metals are almost never used in practice, for example. gold 9999 wears out very quickly; The exception is electrical copper and zinc for galvanizing, the cleaner they are, the better. Most often they use the so-called. eutectic alloys; eg steel is a eutectic of iron and carbon, and duralumin is a complex eutectic of several components. If the melt is allowed to sit, the structure of the eutectic in the casting will change and the finished product will be spoiled. The holding time is especially critical for bronze and brass: they need to be cast immediately, as soon as the play of the melt in the crucible apparently changes and becomes calmer. Remember how the engineer Telegin in A. N. Tolstoy’s “Walking Through Torment” was worried that the bronze would not wear out?

In relation to the manufacture of a homemade crucible, degassing of the melt during exposure is significant in that at this time it (the crucible) experiences significant dynamic loads from bubbles of released gases and/or the play of the melt itself. That is, make the crucible withstand a large amount of thermal deformation and, if recovery is required, a small amount. Its material must also be viscous enough to withstand shock waves from bursting bubbles and shocks from melt jets. It is this circumstance that explains the low durability and reliability of homemade graphite crucibles (see below).

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What to make from

Melting crucibles are made (see figure below):

Crucibles for melting metal from various materials

Their comparative characteristics are as follows:

Note: graphite, cast iron and steel crucibles for use in induction furnaces (see below) are completely unsuitable, because completely absorb EMF energy.

About graphite crucibles

Graphite crucibles are made either turned from massive natural graphite (expensive), or sintered at high temperatures from graphite powder (cheaper, but still not very cheap). Hobbyists often try to make “graphite” crucibles from ground graphite with a kaolin binder, etc., but what they end up with is not graphite, but overly graphitized ceramic crucibles - fragile, withstanding no more than 10 melts and spoiling the metal due to excessive release of atomic carbon by finely dispersed graphite . A more or less rational way to use ground graphite in amateur crucible melting is to make a tabletop mini crucible furnace from it for ceramic neutral crucibles, see fig.

Graphite Mini Furnace for Jewelry Crucible Heating

Cold welding for assembling this furnace should be used at a temperature of at least 800 degrees - the cheeks, which conduct electricity well, do not heat up above 400 during one melt. Graphite powder will not heat up much more without a crucible, but when the crucible is pressed into it, it will be hot spot over 1000 degrees due to compaction of the powder under the crucible.

If gold is melting, then after the melting is completed and the furnace has cooled, the graphite powder is poured out and shaken, because it gets baked. To melt silver and cupronickel, the powder is removed and shaken after 3-5 melts, so the furnace heats up faster. In any case, to maintain a reducing environment, the furnace is covered with a mica lid during melting.

Safety precautions

To carry out all operations with metals heated to very high temperatures, it is necessary to take care of your safety and minimize the impact of the process on health. It is worth remembering what metals brass is made of, at what temperature a particular sample melts and how it is achieved. Here are some tips:

1. Use protective gloves and clothing made from materials that are difficult to burn - wool, cotton and others. You should not use synthetic clothing, as it can catch fire very quickly.
2. Take care to protect your eyes and face with glasses and masks, as an accidental drop of molten metal can cost you your eyesight or cause a serious burn to your facial skin.
3. Casting must be carried out in a place with sufficient ventilation, as during the casting process substances are released that, if they acquire sufficient concentration, can cause great harm to your health.
4. In order to minimize the risks of arson or accidental ignition of nearby objects, you can cover the surface on which the stove will be located with an asbestos sheet. Again, do not forget about good ventilation.

By following these guidelines, you can safely and effectively handle hot or molten metals without fear of harm to yourself or others.

Heating methods

If you need to melt more than 150-200 g of metal at a time, then you will need to build a crucible furnace next to the crucible, otherwise it will be very difficult to achieve homogeneity of the melt and high quality casting. The exception is low-melting and easily recoverable lead: up to 20-30 kg of it can be melted at a time at home. A relative exception is zinc for hot galvanizing; its melt in a crucible without a furnace can be up to 2-2.5 kg, but borax must be sprinkled on top of it so that the surface of the melt is completely covered with its fluidized layer. Steel fasteners are thrown into the melt through a layer of borax.

The optimal method in all respects for heating the crucible in a furnace is with gas, pos. 1 in Fig., but a gas crucible furnace is a rather complex structure, although it can easily be made independently. The most suitable crucible for a gas furnace is a graphite ceramic crucible, because its material has fairly high thermal conductivity. If there are particularly high requirements for metal purity, it is better to use a neutral ceramic crucible. When lower for fusible metals - cast iron, as it conducts heat better and thereby saves fuel. Graphite crucibles are placed in a gas furnace only if strong reduction of old oxidized metal is required, and the danger of carburization is insignificant, for example, when melting silver extracted from the earth for refining

Methods for melting metal in a crucible

For low-melting metals, the electric crucible furnace, pos. 2; it may be the so-called ohmic (with heating by a nichrome spiral) or induction, with heating from an electromagnetic oscillation generator, see below. Only ceramic neutral or, to a limited extent, graphite crucibles are suitable for induction furnaces.

If the crucible contains more than 2-2.5 kg of metal, then according to safety rules the crucible furnace must be made tiltable (item 3), because and 1 kg of melt spilled on the floor is already a big disaster. On the contrary, it is preferable to heat metal in small jewelry crucibles without a furnace, directly with the flame of a burner, pos. 4. In this case, the crucible is held throughout the melting process with a special spring grip, pos. 5 and 6.

Note: silver and its alloys, as well as lead on sinkers, can be melted at home in quantities of up to 15-20 g, using instead of a crucible... a food-grade stainless steel spoon, see fig. on right. For safety, then it is necessary to make gaskets for the jaws of the vice with longitudinal cuts under the handle of the spoon. The flame is exclusively gas; gasoline can burn a spoon.

Electric heating

Ohmic crucible furnaces are mainly used for smelting lead or tin. For more refractory metals, they turn out to be uneconomical, but up to 20 kg of lead can be melted at a time in a home crucible electric furnace; how to make your own electric crucible for melting lead, see for example. video:

Video: electric crucible for melting lead

Melting aluminum in a crucible turns out to be more profitable by induction due to its high electrical conductivity, but this trick no longer works with copper - its temperature and latent heat of fusion are much higher. With the induction melting method, the metal is heated by Foucault eddy currents, for which the crucible with it is placed in an EMF coil of thick copper wire, powered by alternating current from an electromagnetic oscillation generator. How to make a generator with your own hands for inductively heating small amounts of metal, for example, for trinkets, is described in other materials, or, for example, see next. video guide.

Video: DIY induction heating

Induction crucible furnace for aluminum melting

With an increase in the amount of melted metal, not only does the required power of the generator increase, but its optimal frequency also decreases, this affects the so-called. surface effect (skin effect) in metal. If 100-200 g of aluminum can be melted into EMF from any homemade generator for inductive heating, then installation of 1.5-2 kg of duralumin or magnesium alloy is already a solid structure, see fig. on right. If you intend to work with aluminum, then think carefully - is it worth building something like this? Wouldn't it be easier to use a mini gas furnace for melting small quantities of aluminum alloys, see for example. video clip

Video: mini furnace for melting aluminum

Physical features of melting homogeneous metals

Brass is a multicomponent alloy based on copper and zinc. It may also contain some other components - tin, lead, iron, nickel, manganese. Copper acts as the main substance, while additional components improve the physical properties of the material (strength, elasticity, electrical conductivity, corrosion potential). Melting of single-component and multi-component alloys has many differences. Therefore, before considering the issue of melting brass, it is necessary to consider the features of melting a homogeneous metal based on copper.

In physics, smelting is a procedure in which a solid metal turns into a liquid state. To melt copper, it must be heated to a temperature of 1.085 degrees Celsius. Typically, heating is carried out with a small temperature increase (~1150 degrees), since in practice copper alloys with the addition of alloying substances are often used, which increases the melting point.

Heating at the chemical-physical level

1. Copper atoms are in a solid state before heating. On a chemical level, this means that they form a strong crystal lattice that is resistant to deformation and retains its shape upon impact.
2. When heated, the potential energy of copper atoms increases, which leads to a deterioration in the strength of the crystalline structure of the material. However, the material retains its hardness because the crystal lattice is not destroyed (although it becomes less dense).
3. When the temperature reaches 1.085 degrees, the copper atoms receive an excess amount of energy, which causes the crystal lattice of the alloy to disintegrate. At the physical level, the alloy changes from a solid to a liquid state.
4. Now several situations are possible. Let's consider the first situation. If the material continues to be heated, it will retain its liquid state. At a temperature of 2.567 degrees, copper goes into a gaseous state (that is, the liquid begins to boil). In metallurgy, evaporation of copper is very rarely performed, since it has no practical benefit.
5. But another situation is also possible. If liquid copper is not heated after melting, then the liquid will gradually begin to cool. This will cause the material to return to a solid form. At the chemical level, the crystal lattice will re-form.

One simple conclusion can be drawn from these theoretical calculations. For one-component compositions, the crystallization temperature and melting temperature are the same. In practice, it is simple to regulate the melting procedure - you just need to reduce or increase the temperature of the fire. During work, it is also necessary to monitor the distribution of fire over the entire area of ​​the metal object. If the temperature distribution is uneven, some components will be in a liquid state, while others will be in a solid state.

Making crucibles

Now it's time to make your own melting crucible. From the above it is clear that it makes sense to make crucibles with your own hands:

There is nothing special to say about steel crucibles - they are just a steel vessel with a welded handle. Steel crucibles are used for melting low-melting metals; sometimes - zinc for hot galvanizing with quality up to 3+. Steel crucibles for lead, tin and zinc are only suitable for melting one specific metal, because... after 1-2 melts they themselves are covered with it from the inside.

Crucible markings

Crucible (VIII-IX centuries). Archaeological excavations of the 1970s, Kamno settlement, Pskov region. State Museum of the History of St. Petersburg

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Each crucible has a brand (number), and this number indicates the capacity of the crucible. Crucibles are marked from 1 to 300. In metallurgy, as the most widespread industry that consumes crucibles, one conventional unit of capacity (1) is taken to be a volume equal to 0.142 dm³ (or the specific capacity of 1 kg of bronze), provided that the crucible is filled with metal to 85 %. For example, the capacity of a crucible of grade 20 corresponds to 2.84 dm³, or, in other words, 20 kg of molten bronze.