Archive for November, 2005

Applying glaze

You may apply glaze in several ways. If only one or two pieces are to be glazed, the glaze may be brushed on. Use a large, flat brush and plenty of glaze, that is, do not scrape off excess glaze from the brush, but keep it well loaded. Make broad strokes going in the same direction until the piece is covered. Work quickly and do not attempt to go over

Illustration 43
An even color results when a dry glaze and colorant are ground with mortar and pestle.

the glaze while it is wet. When the first coat has dried, brush on a second coat, applying the brush strokes in the opposite direction. This will help insure an even coverage. Do not worry about the brush marks as they will not show after the piece has been fired; your main concern is to make sure the piece is covered well. Although glazes flow out and even themselves when heated, they will not ordinarily flow enough to cover any spots left unglazed.

Two coats of glaze are usually sufficient if the glaze has been applied heavily enough. If the glaze has been applied lightly, three coats may be necessary. Too light an application will result in a dull, rough finish, while too heavy an application may chip or blister off in the firing. A little experience should enable you to determine the correct application.

You may, of course, buy liquid glazes ready to use, but glazes sold in liquid form are generally brushed on, an expensive and time-consuming method of glazing if you plan to do much ceramic work. Most ceramists purchase their glazes in powdered form, add water, stirring the mixture to the consistency of thin cream, and then screen the mixture through a 100 mesh sieve.

Glaze must be thoroughly mixed and some ceramists grind the dry ingredients in a ball mill, a jar holding balls or pebbles which, when rotated, cause grinding. However, this is not necessary if you buy the glaze materials from a ceramic dealer because they have usually been ground fine enough so that thorough mixing and screening are sufficient.

If you spray or dip you need only add water to the dry glaze, but if you brush the glaze on, you will have to add a binder, such as gum tragacanth, molasses, honey or heavy syrup about 5% is the usual amount. You may find that after the glaze mixture has been left standing for awhile, it tends to settle and harden at the bottom of the container, making it difficult to stir. To help prevent this settling, add a teaspoon of Epsom salts to each quart of glaze mixture.

If you are adding colorants to a glaze, it may be necessary to grind the dry ingredients since some pigments, even after mixing and screening, will produce streaks of color in the glaze which may or may not be desirable. When a perfectly even color is desired the dry glaze and colorant should be ground either in a ball mill or by hand with a mortar and pestle.

Slip glaze

We have already mentioned the possibility of using a low firing slip as a glaze on higher firing ware. This type of glaze is known as slip glaze. There are prepared slips, such as Albany slip, that may be used for this purpose. Slip glazes are widely used commercially on stoneware crocks and porcelain insulators.

Speckled glaze

A speckled glaze is made by adding to any glaze a granular material that does not melt. These grains remain in suspension during firing and result in definite specks of color. Red grog can be used successfully or you can purchase the prepared specks from your ceramic dealer and simply sprinkle them in the glaze.

Crackle glaze

A crackle glaze is actually a defective glaze, one which does not fit the clay body properly and causes a series of very fine cracks known as crazing. However, it is an extremely effective finish when the pattern of crazing is deliberately accented by rubbing a stain into the tiny cracks. Most glazes can be made to craze by adding a soft flux, such as glass flux, borax or glass cullet, which causes the glaze to shrink more than the

Illustration 41
A clear crackle glaze, applied over an underglaze decoration, makes an effective finish.

clay and thus produces crazing. A crackle glaze, unless it is used on a vitreous body, is not waterproof.

Jewel glaze

A jewel glaze is merely an extremely soft glaze which will pool when used in large amounts. When it cools it becomes a thick layer of glass. Although this type of glaze will craze because of the high percentage of flux, some very beautiful jewel-like effects can be obtained.

Illustration 42
Because of its fluidity, the jewel glaze has run off the raised lines of the design and has crazed.

OPAQUE GLAZE

An opaque glaze is one which covers the clay color completely. To make an opaque glaze, add approximately 10% tin oxide to your clear glaze, or you may use 20% zirco-pax which is sometimes easier to obtain than tin. Opaque glazes may be colored in the same manner as clear glazes, but because of the whiteness of the glaze the resulting colors will be pastel.

SOFT GLAZE

A glaze to which enough flux has been added to provide a relatively low melting temperature is called a soft glaze. Such a glaze will not remain stationary, but will flow freely and, if applied heavily, will drip or pool. You can soften any glaze by adding approximately 15% borax or lead, although lead is dangerous to handle and sometimes affects the color of the glaze. Many interesting and accidental effects can be obtained with soft glazes, but they are somewhat unreliable since it is usually difficult to arrive at the same effect twice.

HARD GLAZE

As opposed to a soft glaze, a hard glaze is one which is stationary and does not move to any great extent. For glaze decoration where any degree of design is desired, it is best to use a fairly hard glaze that will stay put and not run every which way. To harden a glaze add approximately 5%-10% flint.

SALT GLAZE

A salt glaze is, believe it or not, simply that a glaze made from salt. The salt is thrown into the kiln when the temperature has reached its highest point, forming a mist

Illustration 40
A soft colored glaze sinks into the incised design of this ash tray.

which settles on the surface of the ware. When the soda in the salt combines with the clay, a very hard orange-peel textured glaze is formed. Salt glazing works best at temperatures of cone 4-cone 9 and can, therefore, be used only on high firing bodies such as stoneware or porcelain. This is not the most practical kind of glazing for the studio ceramist since it must be done in a special kiln reserved for the purpose, because, along with the ware, the inside of the kiln will be salt glazed.

Using your clear transparent glaze as a base, it is easy to produce a colored transparent glaze by adding oxides, stains or underglaze colors to the clear glaze. A colored transparent glaze gives much the same effect on ceramic ware as a water-color wash on paper; that is, any designs underneath will show through since the glaze is transparent, but will be tinted by the color in the glaze.

With underglaze colors used as a colorant for your glaze, the amount to be added depends upon the depth of color desired. Here you will have to do a little experimenting to find the exact shade of color desired. The colors may be added in liquid or dry form.

There are also ceramic stains which can be used for coloring purposes. Check with your ceramic dealer for a list of the many stains available.

Metal oxides are used by many ceramists as coloring agents. Such oxides are the basic source of ceramic color and are used in the prepared underglaze colors, overglaze colors and stains. Here is a list of a few of the most commonly used oxides:

Cobalt-Cobalt is the most intense of all ceramic colorants. A very small amount (1% or even less) will produce a strong cobalt blue. Too much cobalt will result in a very deep color that is almost black, so use this oxide sparingly.

Copper-This oxide produces various shades of green and in some glazes will produce a rich turquoise. If more than 6% copper is used it will cause the glaze to flow, since copper oxide has a fluxing action. Too much copper will also make the glaze dull and metallic which, incidentally, is a beautiful effect.

Iron-Iron is usually found in clay itself, producing warm, red tones. The ceramist uses red iron oxide to produce glazes ranging in color from amber to dark red-brown, depending upon the quantity used. The usual percentage is between 5% and 10%.

Uranium-Uranium produces reds and oranges of varying shades. Between 5% and 8% is generally used to color a glaze.

Illustration 39
A rutile glaze that produces a crystalline effect was used on this handsome ceramic
fish. (Courtesy of Florence Cox.)

Too much uranium will result in a black glaze. You may encounter some difficulty in obtaining uranium now for the obvious reason that it is in great demand for other, less artistic, purposes.

Titanium-This is also referred to as rutile and produces interesting crystalline effects, especially when used with copper or cobalt.

Clear transparent glaze

There are so many kinds of glazes that you could spend a lifetime simply experimenting and learning about them. The first kind to be considered is the clear transparent glaze. This glaze is used on ware that has been decorated with under glaze colors or engobes. Such ware requires a colorless, transparent covering that will allow the decoration to be seen.

Clear glaze may have a glossy finish, it may be extremely matt or it may have a finish that is neither a high gloss nor a dull matt, but a satiny compromise. If you should want to make your clear glossy glaze

Illustration 37
A clear transparent glaze has been applied over an underglaze decoration. (Courtesy of
Florence Cox.)

Illustration 38
A colored transparent glaze provides the only decoration for this figurine. Oxides, stains or under-glaze colors added to clear transparent glaze will produce a great variety of colors.

into a clear matt glaze, add zinc oxide. The addition of approximately 20% zinc oxide to a glossy glaze in its dry form will produce a semi-matt finish, or satin matt, as it is referred to by some manufacturers. A satin matt glaze provides the most attractive finish for any ware that has been decorated with under glaze colors. To obtain a very dull glaze with no sheen at all, add approximately 35% zinc oxide to your dry glossy glaze.

Glazes

To approach the subject of glazes from a chemist’s point of view is unnecessary for a ceramist. A small amount of technical information about the chemistry of glazes will provide a fundamental understanding of glazes and glaze techniques.

What is a glaze?

Broadly speaking, a glaze is a glassy finish used on ceramic ware to enhance its beauty and to render the ware waterproof. This glass covering or glaze contains two of the ingredients found in clay: silica and alumina. As a matter of fact, clay by itself could be used as a glaze. When clay is fired beyond its maturing point it will eventually melt and, when cooled, it will harden into a glass-like material. As an illustration, examine one of your clay pyrometric cones which has been overtired and has melted. You will notice that what was once a clay cone has melted and cooled into a shapeless, shiny, hard mass.

If this clay were applied in liquid form to higher firing ware it would melt and then cool and harden on the ware into a glaze-not a very good glaze for it would be rough and bumpy in spots, but a glaze, nevertheless. Clay as such is not ordinarily used as a glaze, mainly because the melting temperatures of most clays are so high as to be impractical for general ceramic purposes.

Something must be done to lower the melting temperature and for this reason an ingredient called flux is added. Some flux is used in the making of clay bodies to lower the maturing temperature, but if enough flux is added the clay will melt at the temperature at which it previously matured. Glaze, therefore, is composed of three ingredients: silica, alumina and flux.

All glazes-colored, clear, crackle, matt, glossy, and so forth-may be purchased ready made. However, it is cheaper to start with a clear glaze and add colorants or other in gredients to produce the kind of glaze you want.

Commercial molds

You can buy molds from ceramic studios or mold manufacturers who will be glad to send you their catalogs. There are several things to look for when selecting a mold. Expense is always a major consideration and one-piece or two-piece molds are usually less expensive than molds consisting of many pieces, depending, of course, upon the size of the mold. It is a good idea, especially if you do not intend to buy a great many molds, to look for molds you can use in various ways. For example, many cast pieces can be cut in half to make a completely different item. Plain pieces can be pinched and flared, incised with designs, or given handles that will transform them into new pieces. You will be able to manage with fewer molds if you select those from which the cast pieces can be individualized easily.

Slip casting

It is much safer to buy prepared slip in liquid form, ready for use, than to make your own slip. If you buy the slip in powdered form, be sure to ask the dealer if a deflocculating agent has been added. Your mold must be dry before casting or the slip will not set-the drier the mold, the quicker the slip will set. To prepare the mold for casting, first remove any dust or dried particles of slip (if the mold has been used before). A section mold of two or more pieces must be securely tied so that it will not open or slide out of position. Use heavy twine or, better yet, rubber bands cut from an old inner tube. Place the mold on a level surface for casting so the slip will fill the mold evenly.

Make sure you have enough slip to fill the mold and pour it in an even, steady stream up to the very top. Leave the slip in the mold for about fifteen to forty-five minutes, depending upon the size and dryness of the mold and the consistency of the slip. Light slip will take longer to set than heavy slip. Experience will enable you to judge the length of time required for the slip to set. As the mold absorbs the water, the level of

Illustration 33
When slip casting, pour slip into the mold level with the top.

Illustration 34
When the slip has set, pour the excess out.

the slip will drop and you will be able to determine the wall thickness of the piece. An average piece should have walls about 3/16 of an inch thick. When the walls have reached the desired thickness, pour out the excess slip and turn the mold upside down to drain. It is a good

Illustration 35
Turn the mold upside down to drain for about two hours.

Illustration 36
Remove the cast piece from the mold.

idea to place a wedge under one end of the mold to allow the slip to run off in a single stream, rather than in globs. Let the mold stand for at least two hours, although this length of time, too, will vary according to the size and dryness of the mold. Here again, experience is the best teacher.

Turn the mold right side up. The cast piece should be leather hard by now and will have shrunk away from the sides of the mold. Open the mold and remove the piece. If it tends to stick in the mold, a slight jarring will loosen it. The average mold can be poured about three or four times in an eight-hour period. It should then be allowed to dry, open and at room temperature, for at least sixteen hours before it is used again.

Fettling

Pieces that have been cast from a mold of two or more pieces will have slight ridges at the seams where the sections of the mold join. These “fettles,” as they are called, must be removed before the piece is decorated. Trim them away with a knife and then remove the remaining traces with a damp sponge. The spare is also removed in this manner. Do not use sandpaper for fettling, for the particles of dust will cause trouble when the piece is glazed.

For more complex shapes that cannot be pulled out of a one-piece mold it is necessary to make the mold in sections. Molds can be made in as many as fifteen or more pieces, but we will concern ourselves here with a mold consisting of two sections. As an illustration we have used a candlestick holder that would not pull out of a one-piece drain mold because of the undercuts on the holder.

The first step is to divide the holder (visually, of course) into two equal sections. With a pencil or scriber, draw a line from the top of the holder straight down through the center to the bottom. Use a T-square to make sure that the bottom and top points are perfectly perpendicular. Do the same thing on the opposite side of the holder, so that the holder is now divided into two equal parts from top to bottom.

Since the candlestick holder is hollow, it must be filled with plastic clay, level with the top. At this point it is generally a good idea to add a clay plug or cork on the clay-filled opening to form a spare or waste rim. The spare will allow for the sinking level of the slip when the piece is cast. Start forming the spare about ? of an inch from the edges of the holder and build it to a height of 2 or 3 inches. This extra clay piece you have added will now become part of the mold, and the spare will be trimmed off each cast holder when it is removed from the mold.

After you have formed the spare, place the holder on its side on a board or other flat base and embed it in clay up to the dividing line so that half of the holder is actually buried in clay. Grease the candlestick holder and base, build retaining walls and pour the plaster into the walled area in the same manner as described for the press mold and one-piece drain mold.

When the plaster has set (in about thirty to forty minutes) remove the walls and turn the mold upside down. Remove the base and the clay in which the holder is embedded. The half of the candlestick holder which was originally exposed is now encased in plaster and, after any uneven or bumpy spots are

Illustration 26

Illustration 27

Illustration 28

Illustration 29

Illustration 30

Illustration 31

26. For a two-piece drain mold, divide the candlestick in half with a pencil line.
27. Embed it in clay up to the line and add the clay spare.
28. Grease the holder and clay, build a retaining wall and pour in the plaster.
29. When plaster sets, turn the mold upside down and remove the clay and base.
30. Make keys in the first section of the mold.
31. Grease the mold and holder, erect another wall, and pour plaster for second section of mold.

smoothed out with a scraper and sandpaper, the first piece of the mold is completed. The holder may be left in the plaster.

It is now necessary to make keys in the mold so that when the two sections of the mold are put together they will lock in place. The keys may be of any shape, but round keys are the easiest to make. Simply rotate a spoon or rounded dinner knife in one spot on the face of the mold until a depression of about 1/4 to 3/8 of an inch in depth is reached. Make at least four keys, two on each side of the mold. (It’s a good idea to make one hole a different size or shape for ease in assembling the finished mold.) Brush off the excess plaster chips and you are now ready to make the second section of the mold.

Illustration 32
The completed mold.

Grease or soap the entire face of the mold (including the keys) and the candlestick holder at least twice, allowing an interval of thirty minutes between applications. Build the retaining walls again and pour plaster into the walled area. When the plaster has set, remove the walls and pry open the two halves of the mold, which should separate easily if they have been properly greased. Remove the candlestick holder and put the two sections of the mold together. Allow the mold to dry for a day or two before using it.

A one-piece drain mold can be made over any piece whose widest point is at the top, provided that the rest of the piece contains no undercuts or grooves that would make it

Illustration 20
The first step in making a one-piece drain
mold: fill a bowl with plastic clay level with
the top.

Illustration 21

impossible to pull the piece out of its plaster encasing.

To make a one-piece drain mold over a small bowl, first fill the bowl with plastic clay level with the edges and fasten the bowl face down on a board or other flat base. With clay and a modeling tool, fill in the small undercut that may exist on a bowl with a rolled edge, such as the one illustrated in the photos. Now erect a retaining wall around the bowl. If the bowl is fastened to a round base, the wall can be made very easily with a strip of cardboard or linoleum held together with clothespins and rubber bands. Mix the plaster and proceed in the same manner as with the press mold, again making certain that you have mixed enough plaster to fill the walled area.

Illustration 22

Illustration 25

21. Place the bowl face down on a flat base and grease it.
22. Erect a linoleum wall around bowl and base and mix the plaster.
23. Pour the plaster into the walled area.
24. Remove the wall, base and bowl when plaster is set.
25. The completed one-piece drain mold.