Colorobbia - Hobby Colorobbia FAQ

Here follow the answers to the most frequent questions asked. However, please do not hesitate to contact us for further clarification or if you cannot see your question on the list:

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What is clay for ceramics?

Which are the Hobby Colorobbia clays? What are they like? At what temperature do they need to be fired?

Which are the principal methods of working with clay?

Which are the principal problems you come across when firing clay?

What is a glaze? Is it a transparent frit? Is it a colour for ceramics? And, how is it applied?

Which are the principal problems you come across when firing glaze?

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What is clay for ceramics?

Essentially, clays are a form of pulverised rock and are generally classified into two types according to their geological origin:

Primary Clays – these have not been moved by the force of nature and are therefore the purest and the nearest to the theoretical formula and the whole group practically evolves round a single type of clay, Kaolin, which is the basis and the principal component of our white earthenware.

Secondary Clays – on the other hand, these are clays which have been transported by the force of nature (sedimentary origin) and have certain impurities incorporated within, such as quartz, mica, alkalines, carbon-like matter and above all iron –oxide from which we get the typical red colour of our clay.

Chemically, they are basically composed of hydrated silicates of alumina, in other words, alumina silica combined chemically with water, and contain numerous impurities which can considerably change their characteristics (colour or firing temperature).

These mineral substances called clays have a fundamental characteristic in common: their plasticity when they come in contact with water by which we can manipulate them in order to achieve our objects. In ceramics the clays obviously need to be fired, and the following classification takes into account the various temperatures at which each clay better expresses its particular characteristics:

Fusible clays: they remain porous in firing up to a certain temperature ( up to 980° for red clays, 1040° for white clays); once beyond these temperature limits, they lose their porosity as they begin to melt, or rather, they no longer absorb any glaze that has been put on them and they tend to deform. They are quite plastic and contain a more or less high percentage of calcareous matter and calcium carbonate; in the case of red clays, also iron oxides are present with a percentage that can reach even 8%. There are also organic substances and various impurities present, such as fossil residuals, chalk etc.

Glazeable clays: these clays remain porous up to a certain temperature after which they become compact, that is, they vitrify and maintain this characteristic for quite a wide range of temperatures, thus allowing the realisation of objects that no longer absorb. The firing point for stoneware varies from 1120° to around 1200°, while as porcelain can reach 1280°.

There are also the refractory clays, reaching a firing temperature of around 1500°, whose use is however destined only for technical and industrial material.

Clays in their original state when extracted from the quarries are not generally useable but are always ‘corrected’ in order to obtain a workable product.

Normally clay bodies of our production are obtained from a mixture of clays and raw materials of various nature, conveniently homogenised together. On extraction from the quarry, usually by mechanical means, the clay is stocked in great quantities for at least a year to mature and expose it to various atmospheric agents: in fact, in this way, the organic substances within the clay begin to decompose, making it more plastic, and will eventually be almost totally eliminated until we have a useable object. Successively, the clays are decanted into large tubs of water in order to expel the salts within along with any residual organic matter.
The resulting slip is mixed with other clays treated in the same way and various components and correctors then passed through various sieves with finer and finer meshes. In this way, we have a homogeneous slip with the required characteristics It is then filtered in special machines called filter presses: they compress the slip until all the water therein is expelled and the final hardness of the clay depends on how much water is more or less expelled.
The last procedure is the outgassing, or rather, the clay is compacted and all the gas therein is expelled and we are left with the block of clay that we all know. By drying the slip we get the dried mixtures used for casting.

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Which are the Hobby Colorobbia clays? What are they like? At what temperature do they need to be fired?

RED CLAYS

APC 5 – AFD 57 – AFD 55: normally known as majolica, this is the traditional body of Colorobbia. It is composed of a mixture of several clays. Its maximum firing point is 980°C.

AFD 65: Red Clay Heat Resistant, that is, clay suitable for the realisation of flameproof objects, orange in colour. Obviously, due to the low expansion of this body, all the glazes used on it will be subject to the phenomenon of hair cracks. Its maximum firing temperature is 980°C.

AFD 70 : a clay of a characteristically intense red, which thanks to its reasonable resistance to sudden changes in temperature can be used in the realisation of objects intended for outdoors or for kitchenware that is not flameproof (oven, micro-wave etc.). Also in this case the maximum firing temperature is 980°C.

AFD 110 : Galestro clay exceeds itself in the realisation of outdoor objects and sculptures. After firing, its inborn characteristics make it come out in white streaks (saline effloressence) and lose the calcites (naturally present in the body), in such a way that it takes on a nice olde worlde effect. Maximum firing temperature is 970°C.

AFD 111 : this has the same characteristics as Galestro AFD 110, but is black in colour.

TFR 1820 : a body with the same characteristics as AFD 55, but with the addition of grog incorporated. Suitable for the realisation of large pieces, including sculptures.

TSC 2070 : a red powder mixture, for the realisation of objects cast in chalk moulds. Its maximum firing point is 980°C.

WHITE EARTHENWARE

TFF 1503 – TFF 1504 – TFF 1505 : a white body coming from a mixture of Kaolinitic clays and calcium and magnesium carbonates. Maximum firing temperature is 1020°C.
TFR 1800 : a body with the same characteristics as TFF 1505, but with the addition of grog inside. Suitable for the realisation of large pieces, including sculptures.

TSC 2050 : a body with the same characteristics as the ones above, supplied in granules, suitable for the realisation of objects cast in chalk moulds by adding the right quantity of fluidiser ( sodium silicate) FSC 2500 or FSC 2510, supplied by us.

REFRACTORY CLAYS AND RAKUCLAY STONEWARE

MHC 257 : an ivory-coloured semi-refractory clay with grog (silica granules) of medium-sized grading (between 0 and 0.5) suitable for the realisation of objects for raku and sculptures, with a good resistance to sudden changes in temperature. For temperatures up to 1100°C.
MHC 747 : a semi-refractory clay with the same characteristics as MHC 257, but with grog of a bigger-sized grading, between 0 and 1.5, suitable for the realisation of very large objects.

MHC 885 : a semi-refractory white clay with added grog of average grading (0 – 0.5), suitable for the realisation of objects for raku and sculptures, with an excellent resistance to sudden changes in temperature and being stoneware, can stand temperatures up to 1300°C.

MHC 886 : a semi-refractory white clay with the same characteristics as MHC 885, but with grog of a bigger-sized grading, between 0 and 1.5.

MHC 887 : like MHC 885, but red.

MHC 888 : like MHC 886, but red.

MHC 889 : like MHC 885, but black.

MHC 890 : like MHC 886, but black.

THE MODELLING CLAY SERIES : coloured earthenware with a very fine grog incorporated. Available in six colours (black, red, pink, blue, green and yellow). Workable in any way and also suitable for sculptures. For temperatures up to 1020°C.

THE PLAYCLAY SERIES : a series of coloured clays that harden without firing. Available in six colours (white, terracotta, black, blue, green and yellow). They can also be fired, in which case the black, green and blue become white and the terracotta and yellow become red. This is due to the fact that the clay is coloured with pigments that are non-resistant to firing.

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Which are the principal methods of working with clay?

SHAPING METHODS

WHEEL – MOULD – SLAB POTTERY – MODINE – COILING – FINISHING

FIRST FIRING OR BISCUIT FIRING

Wheel : shaping on a wheel is the classical system and the widest used in the realisation of ceramic objects. The first thing to do, which is fundamental for the success of the product, is to centre the lump of clay in the middle of the turning wheel: if it is not dead centre, it will spin in an erratic way making it impossible to realise anything; the next step is to make a well in the centre of the body in order to start pulling up the rims of the object and determine the thickness of the base: too thin a base will split on drying and if it is too thick, it may break during the first firing. We can now begin to pull up the sides of the vase until we reach the desired height and make sure that not only the base is centred, but also the mouth of the vase. With a hand inside the lower part of the vase, we press outwards thus creating the belly of the vase, keeping our eye on the thickness of the walls at the same time. Once done, we can finish it off as we wish before proceeding to remove it from the wheel with the steel wire clay-cutter.

Mould : this consists of pressing the clay into pre-prepared chalk moulds, using our thumbs along the whole length of the mould, and more important, trying to maintain the same thickness of clay throughout. The external rim is finished off thicker than the rest with a palette knife or the wire cutter as above, we then wait for a while until the chalk mould has absorbed all the water from the clay so that the object comes away easily to then be cleaned well of any imperfections and/or chalk remains.

Slab pottery : the method is practically identical to the previous, only that the clay to be applied to the chalk shape consists of slabs of clay formed by rollers.

Modine: a method still widely used in the realisation of plates, especially for those of a wide diameter : we need a chalk mould of the plate, on which we place a slab of clay with a slightly greater diameter than the mould after which we begin to make the slab adhere to the mould by pressing on the slab with wet hands, then we pass on to the sizing of it with a special template that corresponds to the mould underneath. Obviously the object will have to be cleaned of any smears and finished off with a sponge. For best effect, we recommend the use of a very soft clay, even softer than that used on a wheel.

Coiling : this technique consists of placing one hand-rolled ‘coil’ of clay on top of another (the ‘coils’ can also be made with our wall extruder), then completely smoothed down.
In any case, all the objects realised must be dried out very slowly, preferably covered with cling-film and never exposed to direct sunlight or heat. Before completely drying, the pieces will be finished off further with the help of special modelling tools. Finally we proceed to the biscuit firing. This phase is fundamental not only to make the object useable in the following stages, but above all to expel from the red clay, for example, all the carbon dioxide found within the calcium carbonate. If this were not so, during the glaze firing stage, the releasing of gasses would give way to air bubbles or little pin-holes on the surface of the glaze, leaving the object badly flawed.

Furthermore, during the biscuit firing we have to be careful that the surfaces of the different objects do not come into contact with each other, as this would impede the complete expelling of the gasses and the stagnation of these gasses would cause the formation of ugly dark-coloured rings due to the oxidisation of the surface of the objects.
The different stages of biscuit firing are roughly the pre-heating up to around 300°, which permits the complete loss of any water left within the objects, then the actual firing at the maximum stated temperature, then the all important cooling, taking into account the various volumes of the objects as any sudden acceleration of this process can bring about cracking and breaking.

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Which are the principal problems you come across when firing clay?

PRINCIPAL FLAWS TO BE FOUND IN CLAYS AND EARTHENWARE:

The formation of bubbles : this is caused by excessive firing and the obvious way to avoid this flaw is by firing at an inferior temperature. Sometimes when firing pieces that have not been sufficiently dried too quickly, any humidity within remains trapped because the excessive firing speed seals the external porosity.

Deformation : this is caused by a too sudden drying process or excessive firing temperature.

Fractures : a kiln that cools down too rapidly, or has been opened at too high a temperature can give way to fractures in the object.

Drying cracks : this can be due to a number of causes, the most frequent being:

Dis-homogeneity in the density of the piece. In fact you can find areas completely dry and others still damp; the tension produced at the boundary of these two areas brings about a crack.

The irregular drying process of a piece with the same density but with areas dried differently.

Re-wetting a piece that has already been dried. It is however a good rule of thumb to dry the pieces slowly and never exposing them to the sunlight or direct heat, even better, cover them with cling-film.

Firing cracks : the most frequent causes are:
   The piece has only been glazed internally or externally with a very aggressive glaze.
   The temperature of the kiln is increased too rapidly within the first 450° of firing.
   The pieces have not been completely dried before firing.

Imperfect adhesion of the glaze : normally the object has been over-fired and does not absorb or the surface has been dirtied (even the simple contact of a sweaty hand can be enough): in this case, use a fine sand-paper and clean any dust from the piece well.

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What is a glaze? Is it a transparent frit? Is it a colour for ceramics? And, how is it applied?

GLAZES, FRITS and COLOURS

TECHNICAL CHARACTERISTICS – APPLICATIONS – DECORATING METHODS
THE SECOND FIRING – AN OUTLINE OF THE THIRD FIRING

The stage following the biscuit firing is that of the glazing; by glazing we generally mean the application of a frit or a glaze onto a biscuit object.
By frit we mean a transparent vitreous coating, normally supplied in powder or liquid form, applied to the biscuit by immersion, air-brush or paintbrush.
Frits are normally divided into transparent gloss, transparent matte or satin-finish, craquelle, coloured.
Firing temperatures go from 920°C for the more delicate frits with a low lead content, up to 980°C – 1000°C for those that are harder to melt.
A glaze is normally a frit, the basis of all glazes, but opaque or with a white base that could be tin, titanium, zirconium or a combination of these, or it could be with a coloured pigment base.
Also glazes, like frits, are divided into gloss, matte or satin-finish, craquelle.
There are also special glazes (like selenium reds, cadmium yellows, or very special glazes like leather-look or metallic), that due to their special effects allow us to get very particular results.
Like the frits, also the glazes can be applied by immersion, air-brush or paintbrush and like the frits, firing temperatures go from 920°C for the more delicate frits with a low lead content, up to 980°C – 1000°C for those that are harder to melt.
Another important factor to be taken into consideration, for powdered frits and glazes, is the density of the application, or rather, the percentage of water to be added to the powder. It is not possible to establish at a glance how much water to add just by the quantity of the powder, but we also have to know the specific weight of the powder particles. Generally speaking, frits can be applied more lightly than glazes which would lose their covering properties. The two main types of decorating are Underglaze and Overglaze or Majolica.
In the first case, the biscuit is decorated by coating it with a layer of frit, while in the second case the biscuit is glazed and then decorated on top of the glaze.
The colours such as the CAV 2000 series, used in the underglaze decoration, have a low-content flux as the gloss will be determined by any successive frit applications, while overglaze colours like the CAV 12000 series contain more flux, having to add gloss on top of the glaze.
Our ready-to-use Hobby underglaze colours are especially suitable for under-colour applications: to be applied on top of glaze, they need a very light coating of a transparent frit applied with an air-brush.
Obviously, it is always possible to add some flux to the colours, being careful not to exaggerate, otherwise they will lose their covering effect.
The so-called ‘Third-Firing’ technique consists of decorating an object that has already gone through a second glaze firing and is therefore glossy (like white porcelain, for example).
Special colours are used that generally melt at lower temperatures than those of the second firing: around 700°C to 820°C.
The need for these colours stems especially from the decoration of porcelain, which goes through a firing temperature higher than 1200°C, temperatures that cannot allow the correct development of the colours required (especially the reds), and so to get a bright decoration, the third-firing technique is necessary.
This technique is also used to apply the golds and lustre metals, being special colours that give off a particular transparency and metallic shine.
Third-firing colours cannot be dissolved in water as this would not permit their adhesion to glossy surfaces, but are diluted with a particular medium (various natural essences, turpentine, water-based mediums) that differ from each other for their viscosity, their easiness of application, and for the speed with which they more or less dry, or in the case of wet oil, do not dry at all.
The precious metals and lustres are usually diluted with turpentine essence.
The above reference to a range of temperatures between 700°C and 820°C depends on the support underneath: in the case of third firing on traditional ceramics or majolica, the temperature will be around 700°C-720°C, being softer than Bone China porcelain that needs higher temperatures of around 760°C-790°C and in turn is softer than Bavarian porcelain that needs its colours to be fired over 800°C; all this makes it possible for the colours to melt and at the same time adhere perfectly to the surfaces they are coating.

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Which are the principal problems you come across when firing glaze?

A BRIEF OUTLINE OF PRINCIPAL FLAWS TO BE FOUND WHEN FIRING GLAZES

CRACKS : these are usually due to an accelerated heating phase or to an accelerated cooling phase: in both cases the firing cycle has to be sufficiently slowed down. Cracks in firing differ from those in the cooling stage, in as much as the glaze tends to get into the crack itself, while as in the cooling stage, the crack remains clean.

UNDER-FIRED BISCUIT : as already mentioned, an insufficient biscuit-firing will not allow the components of the clay to develop all the gasses that will then form in the second firing, provoking a series of pin-holes on the surface of the glaze.

CRAZING : these consist of a series of superficial cracks in the glaze or frit: they are due to an incompatibility between the thermal expansion of the clay biscuit and that of the glaze: generally speaking, this is rectified by making the glazes or frits harder. An artistic use of crazing is craquelle.

PEELING : in this case the glaze literally peels, especially around the edges, and again this is caused by the incompatibility between the clay biscuit and the glaze. When this happens though, being a technical factor different from crazing, instead of hardening the glaze or frit, we have to soften it by adding suitable fluxes, like a craquelle frit.

CRAWLING OF THE GLAZE : when the object is glazed with an excessive quantity, whether it has been immersed or sprayed, in the thicker areas or near possible drops, the glaze can shrink in parts: these are called crawl. In this case we have to further dilute the glaze-water solution, or in the use of an air-brush or paintbrush, reduce the given quantity.

TOO MUCH COLOUR : if a colour is applied excessively with a paintbrush, especially those colours that melt easily, cracks can happen due to the excessive thickness: apply less colour.

GLAZE SHRINKAGE : if the biscuit is not perfectly clean, and there are areas of no-absorption, glaze shrinkage can occur. Make sure that the biscuit is always perfectly clean, even by using a very fine sand-paper on it in order to remove the non-absorbing surface, clean it well from any resulting dust and then proceed to glaze. Be careful, because this type of flaw can be caused simply by handling the biscuit with sweaty hands.

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