Glaze ceramics decoration
There is some question as to whether glaze was discovered and used first in Egypt or in the land of the Tigris and Euphrates. There is evidence that glazed tiles existed in Egypt of 5500 B.C. However, there are remains of clay which show glaze traces which were found in the Mesopotamian and Persian areas, which may have been created as early as those of the people of the Nile. Strangely, glaze did not appear in China until about the time of Christ, and it is believed that it may have been imported there from the West.
The evolution of glaze colours proceeded relatively rapidly. The addition of earth and metallic oxides resulted in brilliant colours and lusters. It is said that as early as the Eighteenth Dynasty of Egypt between 1540-1350 B.C. there were magnificent glaze colours in use; purple, blue, violet, red, orange, yellow, green, and almost pure whites were employed. These colours were painted on beautiful vases and other Egyptian pottery in typical designs, resulting in some of the finest pieces of ceramic art in existence.
The use of glaze spread to Mesopotamia,
Greece, India, China, and later throughout Europe. The history of glaze
evolution in the various civilizations down to the present is a fascinating
study but is of such great scope that it cannot be included except in
a small way in this text.
Aspects of glazes
Glazes are made from the mixture of highly pulverized silica (pure flint and sand) and other earth or metallic oxides. These oxides determine the colour, texture, maturing point, and other characteristics of the glaze. When heated to the proper temperature, the mixture of silica and other oxides melts or "fuses." When it cools it forms a glass-like surface. A very rough and somewhat inaccurate definition of glaze is that it is a type of glass surfacing.
Glaze has several advantages. It forms a nonporous coating on the clay body; in other words, a watertight surface. It provides a durable and smooth surface and is a media for an infinite variety of decorative colour and tactile effects.
Each glaze has a "maturing" point. In other words, at a certain temperature, a particular glaze will fuse and mature. When it matures it attains the maximum brilliancy of colour and forms the desired tactile surface.
If the heat is not brought up to the maturing point, the glaze will fail to colour properly and forms an irregular surface. The colour may be dull, irregular, and unsatisfactory. The glaze may "crawl" or roll up in lumps.
If the glaze is heated above its maturing point it may become "runny"; it may run down, especially on vertical surfaces, or infuse with adjacent colours, in the kiln. The brilliancy of the colour may disappear. The maturing temperature of the glaze must be always ascertained when you purchase it.
As a general rule, the glaze should always have a maturing point less than, or equal to, that of the clay body on which it is to be put. It should never require a higher maturing temperature than the clay body requires. In buying your glazes always specify the type of clay body you are using, to insure that the glaze you purchase has a lower maturing temperature.
We shall not go into a detailed study of the chemical composition of glazes, other than to point out general types. There are the simple lead glazes, alkaline glazes, borax (or fritted) glazes, and matt glazes, each of which have different characteristics for maturing, brilliancy, hardness, colour, expansion, texture, combining power with other materials, and fluidity (flowing quality) under fire.
Certain glazes upon cooling after firing may have tiny cracks appearing over the surface. This effect is called "crazing." This may result from poor correlation between the glaze and clay body employed (different rates of expansion and shrinkage of each). In some cases because of the antique appearance which crazing may give to the surface, glazes are deliberately designed to give this effect even upon maturing properly. Such glazes are called crackle glazes.
Glazes may be applied by dipping, pouring, spraying, or painting on the clay object. Glazes are applied to low-firing clay objects only when the objects are in bisque form, or on surfaces already once glazed. Generally, glaze is not applied to low-firing green ware (leather hard or "bone-dry") because the glaze tends to prevent the moisture from escaping from the clay body upon firing.
The common glazes are purchased dry and are usually mixed with water in a specified proportion for proper consistency. The manufacturer will indicate the proper proportion of water to dry glaze. Each type of glaze has several associated differently coloured glazes which can be mixed or blended to obtain an infinite variety of colours and shades. The unfired glaze when moist is usually an off-grey shade and very seldom has the hue of the final color which it will finally attain after it has matured in the kiln. Therefore, fire the glaze you intend to use on some clay objects in advance, to learn exactly what it will look like after maturing.
Types of glazes
There are transparent or "clear" glazes which are usually applied over engobe or underglaze colours. They may be used over the bisque if the original clay bisque colour is desired to be seen.
As contrasted to clear glaze, there are several kinds of opaque or partially opaque glazes each with its own characteristic maturing temperature and "covering quality." By "covering quality" we mean the degree with which it obscures the colour of the clay body underneath. Majolica glazes (containing tin oxide) give a high gloss and have high "covering" quality.
Enamel glazes have even brighter gloss or shinyness than the Majolica glazes but have less covering quality. Matt glazes have a nonglossy, satiny colour. They impart a beautiful, velvety, tactile quality to the surface. Some matt glazes have the appearance and feel of fine suede.
Outright opaque enamel glazes have the maximum
of covering quality. However, the colour range of outright opaque enamel
glazes is limited.
Crackle glazes, as indicated above, are employed to give a crazed effect to the surface. The fineness of the crazed pattern depends on the firing temperature, the thickness of the applied glaze, and the particular crackle glaze used. After cooling, the crazed surface may be sponged or painted with any one of the various coloured writing inks thinned with alcohol, or with alcohol dyes. The ink will seep between the cracks. The surface is rubbed clean with a cloth to remove the surface ink. The antique effect resulting is quite novel and exciting. Crackle glaze can be placed over nonflowing glazes (already fired) to give an unusual veiled effect.
Fritted glazes are glazes in various colours that are matured at high temperatures, later pulverized, and then mixed with a low firing transparent flux. When the flux melts at low temperature it binds the particles of the fritted glazes and gives practically the same result as if the object were fired originally with the high firing glaze. The use of fritted glazes has the advantage of permitting this type of glaze to be used on low-firing clay objects to obtain colours and effects which are otherwise unobtainable.
There are special low firing glazes available on the market which have met the demand for especially low temperature maturing. These glazes generally have a high gloss. The advantage of using these glazes is that they can be matured sooner, the low temperature preserves the life of the kiln, and economy in fuel for firing the kiln can be effected. These special low firing glazes are recommended for the beginner.
There are numerous special glazes on sale, with semi-matt, antique, and other visual and tactile effects claimed for each, which can be experimented with for variety.
All the glazes (except the transparent) come in a large variety of colours, which can be further blended to increase the range of hues and shades.
Various companies which sell glazes may specify the thickness with which glaze is to be applied to the bisque. It is obviously difficult to measure such thickness; it is estimated by eye, and such estimates come with experience. The thickness of the applied glaze will depend to a large extent on the porosity of the bisque. The finer the bisque, the less thick need be the applied glaze.
The manufacturer will specify the percentage of water to add to dry glaze for the proper consistency. Each colour is mixed with water in a separate wide-mouthed lidded jar or bowl to a skim-milk consistency before application. After a glaze is mixed it should be strained in a metal sieve (80 mesh cloth) to remove large particles.
Each glaze has a different "flowing" characteristic in the kiln; for example, a simple lead glaze (lead silicate) flows very freely when at the maturing temperature. When applied to an object with slant or vertical sides the glaze will run down leaving such surfaces bare or very thinly coated. Other glazes mature in place and do not flow; and still other glazes flow to a limited amount. A free or partially flowing glaze will flow over and cover up surface imperfections on the bisque-more or less. Matt glazes as a rule do not flow on maturing.
The object's surface should be checked carefully for finger marks and irregularities, and these very carefully touched in with more glaze to avoid bare spots, when using nonflowing glaze.
Application of glazes
Glazes may be applied to bisque or already glazed surfaces by brush, by dipping, by pouring, or by spray gun. When dipping, pouring, or spraying, wear rubber gloves.
Painting: When applied with brush, any kind of artist's paint brush can be used, although ox-hair is preferable. The round brush will "carry" more glaze than the flat brush. However, the flat brush gives a more uniform stripe. In applying the glaze with the brush avoid brushing on top of, and over, moist glaze already applied. The glaze should be applied with one brushful until the glaze runs out; then the brush is dipped in the glaze and carried over to the object, to be applied again. If the glaze is a nonflowing type, do not leave any bare patches, as the glaze will not run over the patch on maturing. For very fine design or painting, use a sable or camel's hair brush. See part II, lesson XI.
If the shape of the object permits (such as a cup, pitcher, or bowl) and different glaze colours are desired on the inside and outside, instead of pouring glaze over the upside down object, the object may be lowered carefully, face up, into the bucket of glaze, almost to the top edge thus covering the outside area (Fig. 37C). The remaining area on the top can be touched in with a brush. The inside is covered with the other glaze as in the previous paragraph.
Spraying: Glazes to be used for spraying are made of a thinner consistency than normal. These glazes must be carefully strained with the sieve.
Spray guns may be purchased which are hand operated (similar to a Flit-gun); or if considerable spraying is contemplated, a motor-driven outfit may be procured. Beautiful two-tone, spattered, or blending effects can be obtained with a spray gun which cannot be accomplished otherwise. With the hand operated type, separate jars which fit the gun may be procured to store different glazes until required. These jars should be capped to prevent evaporation of the water from the liquid glaze. If a motor-driven outfit is used, and much spraying is anticipated, a mask similar to a military gas mask should be procured and used to prevent inhalation of the glaze. The glaze contains silica and often other toxic materials, which are undesirable in the lungs.
The glaze must be air-dried before firing. If minor corrections are desired at this time the dried unfired glaze may be scraped off with a knife. However, the glaze which has run into the pores of the bisque is most difficult to remove. An unfired glaze cannot be concealed by covering it up with differently coloured glaze; because on firing both will fuse and form a visible and unplanned-for mixture.
Comments: If glaze has
dried in storage, the solid remains may be removed, reground with mortar
and pestle, and water added for re-use.
When working with glazes, avoid prolonged contact with the glaze on the bare skin, and also avoid inhalation of the glaze. Wash the hands frequently with water.
Decorating wheel: A decorating wheel can be built, or purchased, which is useful and convenient in glazing, in under-and-over-glaze painting, incising, and engobe painting. It consists of a circular turntable mounted on a fixed base. The better turntables are well-balanced, precisely centred on their bases, and use ball bearings. The object to be worked upon is centred on the turntable, and the turntable rotated by hand. The hand may be supported by a hand-rest while the brush or knife is applied to the surface of the object. The object is rotated by moving the rim of the turntable with the other hand. Stripes may be painted on the object as it is rotated (Fig. 37D). Also, if an object is to be painted with a particular design, the turntable may be turned a little at a time as required, so as to present the desired surface of the object to the painter, who thus does not have to move around the object while painting, nor pick up the object each time to turn it. An arrangement for a plaster bat as the decorating wheel head can be provided, so that plastic clay items can also be used on the wheel, and accordingly decorated, while still moist.
Part I. Underglaze colour painting
A large variety of beautiful designs and hues can be obtained with underglaze colours. Underglaze colours are concentrated stains which are applied to bisque or "bone-dry" clay. If underglaze colour is applied to "bone-dry" clay, the clay is fired to bisque, after which a clear transparent glaze is applied and fired on. If the underglaze colour is applied to bisque, the transparent glaze is applied over the underglaze colour and both are fired on at the same time. The underglaze colours do not have a maturing temperature of their own, but mature at the maturing point of the transparent glaze placed over them.
Some transparent glazes may affect some of the colours of the underglaze, so it is important when purchasing the transparent glaze to specify the type and make of underglaze colours you are using.
Underglaze colours may be purchased dry, powdered, and in bulk. They are also sold in several colours in a paint box similar to a student's water-colour paint box. If many small or several large objects are to be coloured, the underglaze should be purchased in bulk and mixed in your studio or workshop.
The dry powdered colour is mixed in a mortar with water and gum Arabic. There are several commercial underglaze gums on the market, but gum Arabic is generally the base for these. The manufacturer will specify the proportions of water, gum, and colour.
The purpose of the gum is to cause the underglaze colours to adhere to the surface of the object, after the water has evaporated. When the object is fired, the gum vaporizes and disappears without effecting the colour itself. Initially, it is desirable that the student purchase a sample set of underglaze colours.
Comments: The gum Arabic is employed as an agent to cause the underglaze colour to adhere to the clay body. Practically all manufacturers sell gum solutions of the proper consistency. However, you can make your own by adding dry powder gum Arabic to a half pint of hot water until no more gum will dissolve. The gum Arabic solution is then ready for use.
Place a very small amount of dry underglaze colour in the glass mortar, add tap water, and follow with a few drops of the gum Arabic solution. Stir thoroughly with the pestle so that the colour becomes a thin liquid-thin enough to flow easily from the brush.
Hints: You can tell if you have added too much gum solution as too much will cause the underglaze colour to peel and flake off on drying. If not enough gum solution is added, the underglaze colour will powder off on drying.
Clay tiles are made by the slab method, size 6 inches x 6 inches x 1//4 inch thick, and fired horizontally to bisque (resting on all four corners to prevent warping) .
The first step will be to establish the colours as a reference, before attempting to paint a design. Underglaze colours, before firing, appear as a pastel shade of their fired final appearance.
Procedure I: Make two tiles
by the slab method, "bone-dry," and fire to bisque. (Purchased
unglazed white tiles will do equally well.)
A little practice will indicate the proper thickness of application. Paint with deft single strokes, from the tip of the brush-do not "scumble" or wash the strokes in a sidewise motion. A good ox-hair brush is the secret of good underglaze painting.
After the colours have been painted in their squares and become completely dry, the transparent glaze is applied on top with the spray gun, with the paint brush, or by any other common method. The transparent glaze should not be applied too heavily, because when it becomes fused it may run and "pull" the underglaze colours with it. The transparent glaze should just "cover" the under-glaze colours. After the transparent glaze dries, the tile is fired in a horizontal position to the maturing temperature of the glaze.
In purchasing the transparent glaze, always specify the type of underglaze colours you are using. It is possible, if the underglaze colour and transparent glaze are not "compatible," that the glaze may destroy one or more of the colours due to chemical reaction during firing. Generally, the glaze should be purchased from the same manufacturer as the underglaze. The manufacturer will specify which type of glaze should be used with his underglaze colours.
Procedure II. For the second tile, we shall actually paint an underglaze design. Select a design from a wallpaper, a textile design, or a simple colour pattern. We shall paint a copy of it on the second tile. The reason we copy a design in the beginning is so that the beginner can master the handling of the colour medium and not be distracted by the problem of design composition. Copy or trace the design with pencil, or through carbon paper, on the bisque tile.
Mix the various colours you intend to use. Only mix the colours which are on the design and in the quantity needed. The colours on the first fired tile, as reference, will indicate the final depth, strength, and appearance of the colours you are applying. With a clean brush for each colour, start by painting in the lighter colours first, followed by the deeper colours last.
Paint with a deft stroke, copying the design in detail. If any of the colours at any place are so thin that the tile body underneath shows through when the colours dry, paint over the first coat with a thin second coat.
Underglaze colours, when applied, smear quite easily before being fired. It is important, in painting, to proceed in such an order that your hand or brush will not smear across colours already applied. Always paint with the tip of the brush. The colour should flow easily.
Once an underglaze colour is applied it cannot be erased or removed. It is essential that you paint accurately. It is desirable, therefore, to paint your picture first on paper, and use it as a reference when painting on clay or bisque.
Complete the design and let it dry. Apply the proper transparent glaze over the surface of the tile and, after drying, fire to maturity. You can note from the results whether you have painted properly. If your colours are brilliant, you are now ready to paint underglaze colours on any clay object and in any design you desire.
Hints: Some underglaze colours must be fired to a certain temperature to "fix" them before glaze is applied. Other colours may take the glaze directly immediately after drying. Ask your manufacturer or supply house for the proper procedure.
If any of your colours in the paint bowls begin to dry, a few drops of water may be added and mixed in with the pestle. An ox-hair brush is used in preference to softer brushes (such as camel's hair or sable), because the material of the underglaze colour is quite abrasive and wears out the brush rather rapidly.
The brushes can be quickly cleaned by rinsing in clear water. The technique for painting underglaze colours to bisque applies to painting on "bone-dry" "green" ware.
The decorating wheel will be useful if you are painting stripes around your object, or if the object has several sides each of which you intend to paint.
There is a considerable vogue for decorated or painted tiles for use as table tops, heating table pads, or gift items-where your personally painted tiles will be unique.
Part II. Overglaze or China painting
Overglaze colours provide the widest range of colour and shades possible in any type of ceramic colours. They are popularly called china painting colours.
They are always applied over other glazes, never over bisque. This accounts for the name "overglaze." They always mature at a lower temperature than the glaze upon which they are applied. Overglaze colours are the lowest firing glazes of all. Metallic lusters such as gold and silver are essentially overglazes.
Overglaze colours may be painted on top of transparent glazes, majolica, enamel, and outright opaque glazes, all of which have shiny, smooth surfaces. The overglaze colours must be lower maturing than these glazes over which they are painted.
It was fashionable at the turn of the century to send young ladies to Europe to learn "china painting," as a matter of cultural and aesthetic education. Such fortunate young ladies were actually taught the technique of painting overglaze colours on white "blanks." Today numerous artists derive a fine income from hand painting or decorating china "blanks."
"Blanks" are tiles, plates, cups, saucers, bowls, lamp bases, vases, statuettes, etc., usually of porcelain or other high fired ware, which are glazed with a solid shiny white or slightly cream colour opaque glaze. They can be purchased at any ceramic supply or china store.
Overglaze colours are employed similarly as are oil paints; and practically all types of oil painting with their delicate and subtle tonal values, hues, and shades can be duplicated in china painting. The incredibly beautiful painted ware of the most famous china houses in the world, such as Dresden, Delft, Sevres, Staff or-shire, Copenhagen, are, in fact, the products of overglaze painting.
All types of overglaze colours may be mixed with oil and turpentine and employed exactly as are an artist's oil paints. The colours are purchased dry, mixed with sufficient fat oil to dampen them, and then diluted to a thick cream consistency with turpentine. The dry powdered colour and fat oil are intimately mixed on a ground glass slab with a steel palette knife or spatula, after which the turpentine is added and mixed in on the slab. The slab is the painter's palette when all the desired colours are mixed or placed on it.
The colour, when properly mixed, should flow smoothly and easily from a sable or camel's hair brush. However, if there is too much turpentine the colour will spread and run on the china. Test the consistency by brushing on a glazed tile. Too much oil in the mixture may cause the overglaze to blister when fired. Just enough fat oil is added to moisten the dry colour.
Store each of the mixed liquid colours, if mixed in quantity, in a separate capped jar. If the colours thicken in storage, add turpentine to arrive at the proper consistency.
The proper fat oil can be purchased from the company which sells the overglaze colours. Oil of Copaba is often used as the fat oil. The technique of blending and applying the colours is identical to an artist's oil painting. However, colour is applied as in "slick" surface type painting, and not in heavy "impasto" or "dry colour" painting.
In painting on glazed ceramic ware, errors of overglaze painting may be removed by "lifting off" the undesired colour with a palette knife and by rubbing off the balance with a turpentine-moistened cloth. Do not overpaint an error with another colour, as both colours will fuse and blend on firing.
When firing overglaze painting, which uses
oil and turpentine media, the temperature is brought up slowly to about
800 degrees Fahrenheit in an hour's time. During this period the kiln
door is "cracked," so that the oil fumes vaporizing from the
oil in the overglaze colour can escape. After this temperature is reached,
the kiln door may be closed and the ware brought up to the maturing temperature
of the overglaze colour.
There are types of water-soluble overglaze colours for sale on the market which come in a box similar to a water-colour paint box (the same as with underglaze colours). These colours are applied from the box directly with a moist camel's hair or sable brush. They have the advantage of being immediately available for use at the moment of inspiration without requiring lengthy preparation of the colours, as is the case with oil media colours.
It is most important that good quality sable or camel's hair brushes be used. In this part of lesson IX we shall use water-soluble overglaze colours-the simplest for the beginner.
Procedure 1: We shall first "prove" or establish our overglaze colours as we did for underglaze colours in part I of this lesson. Divide a glazed tile into triangles and squares as in part I (Fig. 38A). In the upper triangle of the square, paint in the overglaze colour lightly. In the lower triangle of the same square, paint in the same colour heavily. Do this for each colour in each square. After the tile is fired, it may be used for colour reference for future painting with this type of colour. When using overglaze colours employing oil or other media, prove in a similar tile for the colours of each medium.
The water-soluble colour should be moist but not "runny"; it should not run into adjacent colours. If an error is made in placing the paint at a particular spot, the colour can be lifted with the palette knife and/or by carefully rubbing off with a dry cloth. Be careful not to smear adjacent colours.
Avoid building up the overglaze too thickly. If it is too thick it may peel off when fired. The water-soluble overglaze colours when dry will be approximately pastel shades of the fired colours. The matured colours are far more brilliant and glossy.
When the colours are dry, the tile is fired to the maturing temperature indicated for the particular overglaze, in the same manner as indicated in part I of this lesson.
Procedure II: On the second tile, we shall paint a picture or design. Select some simple coloured picture or design from a magazine, birthday card, or drawing. Copy or trace through carbon paper the outline of the picture or design on the tile. The pencil or carbon lines may show very lightly on the tile. Be careful not to rub them off.
Applying the overglaze colour with the same technique as in oil painting on canvas, paint in the colours. These colours can be mixed and blended wet on the tile. Use the tile made in procedure I above as reference to indicate what the final colours and shades will look like after firing.
The number of colours and designs is unlimited. Different types of pictures, each a little more complicated than the preceding, should be copied on the remaining tiles, for practice. Upon firing, you can determine whether you have estimated the true colours or not, and you can correct for the future.
Once you have mastered the colour and weight of overglaze to apply, you can paint any type of picture, shade, and tone, as you would in an oil painting.
Hints: In commercial overglaze painting, in order to mass produce large quantities of decorated ware, printing is done with ceramic inks, employing die stamping, and decalcomania methods. In each case the media is vaporized upon firing, leaving the silica and nonvaporizing chemical glaze elements to fuse in place. In "decals" the paper and all are fixed to the china, and the paper burns off during firing.
Once you gain facility with hand china painting, an ideal gift can be made by painting simple flower or other designs on white blank plates. Writing or lettering indicating the birthday, wedding day, or other special occasion, together with names of the individuals concerned, plus simple designs, can be inscribed on the blank and fired in, as a very distinctive item.
A great deal of pleasure and quite gratifying results can be obtained by the use of mother-of-pearl or metal (gold, platinum, and silver) overglazes or lusters. If a decorating wheel is employed, a plate or cup may be centred on it, and the brush containing the luster held in place while the object is spun around. Fine or thick stripes may be painted on the rim, or inside of the edge of the object, which, when fired, give the object a very rich look. Writing or painted ornamentation in gold or silver on the blank results in very handsome effects. Specially prepared liquid bright metal lusters, ready for use, are available on the market.
Some of the finer overglaze paintings that have been painted by master artists are found on the best French Sevres, or Dresden porcelain. These delicate and subtle shades were achieved by overglazing very thinly, refiring, again overglazing, and again refiring. In some instances, as many as fifteen firings have been required to accomplish the desired result.