What does sintering mean? Artkimia - Art & Jewelry Studio

Most clays and clays, which are composed of powders, either ceramic or metal or plastic, to reach the solid stage, go through a process called sintering. This process is also widely used in industry and is specific to powder metallurgy.

This process avoids energy consumption, which, moreover, would be very high, being a much more environmentally friendly option. The manufacturing process of sintered components is certified as environmentally friendly, because the material waste is very low, the final product is recyclable and the energy efficiency is better, because the material is not melted. Today, whether we are talking about hard metal parts, such as steel, or we are talking about plastic components, such as those for the automotive industry, this process saves a lot of energy and is an economically attractive alternative.

Sintering of metal clay

As we already know, all types of metal clay are composed of metal particles, an organic binder and water. The binder is an organic plant compound to which a sufficient amount of water is added so that, together with the metal particles, a flexible, malleable clay results. After a part has gone through the stages of modeling, drying and finishing, it must be burned at temperatures specific to each type of metal clay. These temperatures are indicated by the manufacturer and are included in the clay package.

A model of sintering

The sintering process can be better understood if we think of the snow crystals gathered in the shape of a lump. What would happen to this lump if we raised the temperature around it without reaching the melting point? The ice crystals in the lumps would begin to melt on the surface, and the mass of the lump would begin to shrink, but without significantly changing its shape. If we stop the temperature rise and return to the temperature from the moment the lump forms, we will notice how it will become like a lump of ice, solid, compact and glassy. The ice crystals joined together to form a block of ice.

Watch the video that shows us how to sinter metal powders.

First part - before sintering

In the case of dry metal clay, the metal particles are bound together only at certain points, through the dry binder. The table seems to be solid, but the piece can break very easily. At this stage, the metal clay has the same fragility as a dry ceramic vessel.

During combustion, in the first phase, at a temperature of about 350 ° C, you will notice how the piece will start to emit a little smoke and a flame will appear. At this point, the binder burns and turns to ash. At the same time, the piece will "tighten" / shrink slightly. If we were to stop the burning process at this moment, we would have an extremely fragile and brittle piece, because the metal particles are in this phase, "stuck" between them, only by the ash of the burned binder.

Continuing the burning, you will notice how the piece will start to bend (if it is flat) or to shrink seeing with the eyes, and the color will change to a bright orange.

Sintering process - above 650ºC

When we reach the shade of orange, ie above 650 ° C, at the particle level, great changes take place. Starting from the edges of the piece inwards, a molten film is formed on the surface of each metal particle, which will favor the gluing between the surrounding particles. The higher the temperature, the wider the molten surface of the particle is towards the core of the particle and thus the more the piece tightens, leaving smaller and smaller gaps between the metal particles. This is the sintering process.

Time, as important as temperature

In addition to temperature, burning time is also a very important factor. It takes as long as possible to sinter completely to the center of the part. During combustion, either in the oven or with the burner, the area that is first exposed to heat is the surface. Too short a burning time can result in a raw piece inside, so brittle and brittle.

That is why you will see varying temperatures and firing times for the same type of clay. For example: Art Clay 999 - between 650 and 890 ° C for 10 min., 20 min., 40 min. or up to 2 hours)

Therefore, the observance of the heat exposure time favors a sintering, not only at the surface of the part, but also in depth. Like a steak. 🙂

The castings are also slightly porous. After the molten metal has been cast into a mold and cooled, it is not as compact as rolled metal because it has not been forged. Small air bubbles may form while the metal is being molded without being compacted.

Because metal clay (metal clay) does not reach the full melting point, but only the surface of the particles, the final product is more porous than the cast metal. Although the result is a hard metal, it is not a dense metal and because of this it is not as hard and flexible as wrought or cast metal.

In order to obtain the most solid sintering possible, the size of the particles used by the manufacturers also matters. We must also take into account the type of metal or metal alloys used.

Sintering of metal alloys and non-precious metals

Metal clays composed of alloys, such as 925 sterling silver, bronze, steel and all variants of non-precious metal clays, oxidize easily, which prevents the metal particles from bonding. Therefore, these metal clays must be burned in two stages and in activated carbon.

In the first stage, the aim is to burn the binder, which usually requires the presence of oxygen. Therefore, it is not necessary to use activated carbon for this step.

For the second stage, however, the cooled part is buried in a layer of activated carbon, 1-3 cm thick, depending on the manufacturer's recommendations.

Activated carbon reduces the amount of oxygen in the furnace and around the part and thus makes sintering possible.

Small pieces of copper or certain types of bronze can also be burned with the burner, because it consumes a large part of the oxygen in the vicinity of the piece. But at the same time on the surface of the parts will be observed how the oxidation process occurs, and after burning, a thin layer of oxidized metal will exfoliate. The unpleasant part is that, with this layer, part of the texture will come off and thus we risk losing precious details of the piece.

The time interval in which the temperature in the ceramic kiln rises to the desired temperature should be within 40-60 minutes. In English you will find this indication of time of rise in temperature, under the name of "ramp". For example, if the temperature rise is standard, ie as fast as the oven can, the name will be "full ramp". The burning time period in which the temperature is maintained at the desired value (hold time) may differ depending on the brand that sells the respective metal clay. This temperature can also vary depending on each oven.

Contraction during sintering

The shrinkage of the part takes place in three stages: drying - due to the evaporation of water contained in the metal clay, the burning of the binder and, most significantly, during sintering, when the metal particles stick to each other and thus the size of the part is reduced . The percentage of shrinkage of metal clay after firing depends on the type of clay and the manufacturer's brand. It should be noted that the metal clay contracts in volume, ie in all dimensions: height, depth, length, circumference and thickness.

Factors we need to consider when sintering takes place

The movement of metal particles during combustion is influenced by many factors. That is why the support on which the piece stands and the position of the piece are important.

Any element that interacts with the movement of metal particles during shrinkage can cause clogging or uneven shrinkage.

Tip:
To avoid deformation, it is recommended to burn the parts with very different volumes of a piece separately. Only then must they be united by another burning.

Improper support, which does not support the part properly, can cause the volume of the part to collapse.

When burning activated carbon rings or shapes, empty on the inside, we must be careful that the holes in the parts are not completely filled with carbon, as this can affect the reduction process.

Parts with fluid, curved or large shapes, hollow on the inside, must be supported with ceramic fiber wool or vermiculite. The most suitable cotton wool is the biosoluble one, because it is easily dissolved by the body, in case of inhalation.

The shrinkage process is also influenced by the state of the clay in the modeling stage. Fresh metal clay removed from the package contracts differently than rehydrated or recovered clay.

Another important effect on contraction is the burning method. Burning with the burner, done by hand, is an imperfect burning, while burning in the oven, distributes the heat evenly around the piece.

Therefore, it is important to study the piece well before burning it because even gravity has a major effect. For example, if we have a parallelepiped shape, we will avoid placing it with the longest side horizontally. This will cause it to collapse during combustion.

Corina Gheorghe

Corina Gheorghe is the founder and main instructor of ARTKIMIA – Art & Jewelry Studio (Metal Clay). She is internationally known as an artist and teacher specializing in jewelry metal clay and mixed. Corina has taught courses in various countries such as Spain, Belgium, Great Britain, the USA and is a member of the Council of the world organization AMCAW (Alliance for Metal Clay Arts Worldwide). She is the first Certified Art Clay Instructor in Romania and the one who introduced this brand, as well as the term "metallic clay" in Romanian workshops. Below you can find some of the courses taught by Corina. Craftcast - Rose & Gem Embellished Metal ClayTotem AMCAW - Tension Set Rings AMCAW - Sculpture and Carving Techniques

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