There's no limit *to how much you can know about glazes, but even the novice can use them with beautiful results.

Glazes consist of materials which, when subjected to intense heat in a kiln, melt and form a glasslike coating on the surface of the clay.Glazes serve two major functions, utilitarian and decora­tive.In pottery, the glaze gives a smooth surface, prevents food and water from be­ing absorbed by the clay, and is more easily cleaned.Glazing also gives a more pleasing color and texture than the clay and adds greatly to the beauty of the piece.In sculpture, glazes are not necessary for practical purposes, but sometimes they in­crease the excitement and interest of such an art object.

The choice of glazes in general, or of a glaze for a particular object, requires con­siderable knowledge, sensitivity, and ex­perience.For the beginner, commercial glazes which have been carefully checked for satisfactory results are suggested.If possible, secure your glazes and clays from the same supply source because glazes are designed for specific clays and firing tem­peratures.

It is vital that you make sure the glaze you use properly fits the body of the clay.Should the clay shrink more in firing than the glaze, for example, whole sections of the glaze will chip off.If the reverse is true, and the glaze shrinks more than the clay, then there will be wide areas on the surface of the piece left unglazed.Do not expect glaze to hide sloppy workmanship.More than likely, it will accent rather than camouflage flaws and blemishes.

Lead oxide and silica in a proportion of 3.7 to 1 will form the simplest kind of glaze.However, it would be very crude and dull yellow in color.Moreover, it would be too fusible under most heat conditions and would lose its gloss if left in the kiln very long.

Photo from "Craftsmanship in Clay"

Most widely used methods oi glazing large areas are by pouring glaze into item for interior glaze, such as in bottle or jar, and by immersing or dipping entire item into glaze, as with figure at left.Be sure to touch up the spots made by the fingers.

Right, sample ceramic buttons that have been fired with actual glazes to show the finished result are available from most manufacturers of glazes for a few dollars.

If you prefer you can make your own sam­ple glazes rather than buy manufacturer's.Coat a piece of defective greenware with smears of glaze in different colors.Al­ways scratch name of glaze through tile.

To mix your own glazes, purchase a mortar and pestle.Or, for homemade equivalent, use a jar and thick glass swizzle stick.Also handy: a palette knife, glazed tile.

To counteract these faults, alumina must be added.Fortunately, since pure alumina is hard to obtain, all natural clays contain alumina.Thus, clay, which is plen­tiful and inexpensive, is added to glazing formulas.From this simple beginning, pot­ters have developed the beautiful and rela­tively easy-to-use glazes available to hobbyists today.

All glazes contain glasslike or glass in­gredients in their basic formulas, which cause the glassy, shiny finish.Color is im­parted to a glaze by the addition of metallic oxides.The more clay present in a glaze formula, the duller will be the finish.Glazes can be transparent or opaque, shiny or matte.Transparent glazes are usually ap­plied over all objects which have been slip-decorated, so that the colors and de­signs may show through.

Wedgwood ware from biscuit oven is dipped in liquid glaze which has consistency of cream.Water in glaze is absorbed by ware, leaving glaze film.

A transparent glaze is not necessarily clear of color.It may be tinted slightly with a small per­centage of color agents.Opaque glazes are applied on pieces which have no painted decoration, where it is desirable to have a solid color, or to cover the color of the clay itself.Opaque glazes are sometimes re­ferred to as enamels.

If you wish to change the texture of a shiny glaze to a matte or dull glaze, you can try adding varying amounts of the fol­lowing materials: bone ash, up to 5%; titanium oxide, up to 5%; zinc oxide, up to 10%; rutile, up to 10%.The materials should be compounded as silicates.The interesting texture of matte glazes are characterized by the presence of an excess of silica in the composition of the mixture.

Many glazes are formed by the use of basic frits.A frit is melted or compounded glass and its purpose is to permit the use of certain soluble ingredients in the glaze formula.,.Such materials as borax, boric acid and soda ash must be made insoluble before they are used.This is done by frit­ting them—that is, heating them to their molten state.This mass is then dripped into cold water.Upon touching the water the molten drops explode into crystals, which are then pulverized into a fine powder.Fritting is useful in making glazes that give crackle finishes, icelike finishes or bubble effects.In describing the method for making frits, it is necessary to over­simplify.The process requires a goodly amount of know-how and technical ex­perience.This, plus the lack of necessary equipment, means that the beginner is bet­ter off buying commercially prepared frits from his supplier.

Lead glazes are most generally used by beginners because they are easy to apply, fire at a low temperature, and form a dura­ble surface.They offer the common range of colors, but have a yellowish tinge and are not as brilliant as boracic or alkaline glazes, which offer such colors as turquoise, Persian blue, and cherry red.The latter are, however, more difficult to apply.Lead glazes may be purchased from any ceramic house, but as each firm names or numbers its glazes differently, you should order them according to the description or sam­ple shown in the catalogue of a specific ceramics house.

Commercial glazes come in powder form.You will have to mix them with water.Since most glazes contain toxic materials, you should take precautions against breathing in the dust and avoid prolonged contact with the skin, particularly where the skin may be broken.

Usually 65 cubic centimeters of water to 100 grams of glaze will give the consistency of heavy cream.The necessary consistency varies according to the ingredients of the glaze, the porosity of the piece to which it is applied, and the method of application.Experimenting ■with combinations and variations will provide interesting results.

To make a practical transparent glaze, use nine parts Frit No.G24 (manufactured by The O.Hommel Co., Box 475, Pitts­burgh, Pa.), one part Georgia kaolin.To this mixture add about 2% ordinary borax to aid the glaze in flowing more evenly when fired.This glaze should be fired at about 1800 or 1900 degrees Fahrenheit.

In order to color this clear glaze, you may add stains in proportion to the depth of the color you desire, usually between 2% to 20% by weight.

Glaze should be flowed on piece, and not brushed on or brush marks will appear; it is necessary to maintain an even coating or thickness of glaze, particularly when contrasting glaze is used over underglaze.

When brushing glaze over a large area it often is necessary to dry-foot the piece being glazed, in order to keep the glaze on the ceramic item from making contact with the kiln shelves.This also allows you to do away with unsightly stilt marks.

Photos from "Craftsmanship in Clay"

Shown above is most accepted method of glazing interior oi ceramic piece.Pour your glaze into the piece and be careful to rotate the piece as you pour back into the container, to cover all areas.

Glazes may also be tinted by the addition of underglaze colors, grinding them together while they are in a dry, powder state.The beginner, how­ever, will find it much easier to use liquid stains, for two reasons—they are already milled and can be quickly mixed merely by stirring into the glaze.Most supply firms carry a wide range of such stains, enabling the hobbyist to develop his own distinctive colors.

A white glaze or white stain will render all of these glazes opaque.Should the glaze be colored, the addition of an opaquifier will not cause a radical color change in the finished ware.However, additional stain may have to be added to maintain the precise shade you are seeking.About 10% white will be enough to add to clear or colored glaze to render it opaque.

If you want a blue glaze it will be alka­line.To mix a typical glaze, take 64 parts soda ash, 30 parts whiting, 56 parts feldspar and 42 parts silica.Mix thoroughly by mill­ing either with a mortar and pestle or with a ball mill.A ball mill consists of a porce­lain jar which is set in a frame and made to revolve on its axis in a horizontal posi­tion.It is about half-filled with porcelain or carborundum balls, which roll against each other and the walls of the jar and thereby do the milling.

You should weigh the ingredients, grind them in a dry state.Then mix just the quantity you need with water, because any that is left over will have to be discarded.Store the excess powder glaze dry.Stir in the water vigorously so the soda ash, which is soluble, will not harden.If it does, it will be difficult to break up.Do not add too much water because of the soluble in­gredients.Remember that excess water cannot be poured off, since some of the in­gredients would also pour off.Have the exact amount of water you require, other­wise the mixture may be spoiled and you will have to mix the glaze anew.

Now, by adding 4 to 5% black oxide of copper, you will get a beautiful turquoise blue.By reducing the amount of this oxide to 2 to 4%, an Egyptian blue will result.About Vi.% of black oxide of cobalt will bring about a rich sapphire tone.For opaque colors mix in 6 to 8% tin oxide.

Always screen a glaze thoroughly be­fore applying it to a ware.A binder is nec­essary when the glaze is going to be used on bisque to prevent it from fluffing or chipping off.Binders are organic materials such as glue, gum tragacanth, dextrine or gelatine.

Above, bowl is being glazed with spray gun.See that glaze is free-flowing, thin enough to spray easily through nozzle.Always test distance on piece of paper for best and even glaze coverage with spray.Spray booth is used, and mouth and nose should be masked.If you don't want to dry-foot the piece, use metal or ceramic stilt shown.

Binders also help to hold the glaze in a colloidal or suspended state.But other ingredients are usually used for this pur­pose.These include calcium chloride, setit powder and bentonite.Only very small quantities should be used.

A properly prepared glaze is neither too thick nor too thin.If applied to a porous bisque surface, the glaze should be thinner than normal.This is because the water will be sucked out of the glaze very fast, leav­ing a thick coating deposited on the surface.If bisque is hard-fired, on the other hand, the glaze must be thicker than usual.Prac­tice will teach you how much water should be added or poured off.If you need to thicken the glaze by pouring off some of the water, let it settle first for an hour or two and pour off the water at the top.There is little danger of losing any of the in­gredients in the glaze by pouring off this water, since none of the ingredients are soluble and thus will sink to the bottom.Alkaline glazes, as already explained, are an exception to this rule.

It would take a book of many pages to cover all the processes, results, defects and cures encountered with glazes.Only the salient points have been touched upon here.Although the hobbyist who prepares his own glazes should be commended, the be­ginner is much better off buying commer­cial glazes at first.This does not mean that after you have learned the basic techniques, you should not go on and develop your own glazes.First, however, it is wise to obtain instruction from a professional ceramist who will aid you in perfecting your glazing methods.

Ceramic spray booth manufactured by Craftools.Inc..is 2 feet wide, comes complete with motor exhaust, turntable, stand and brackets for gun.

Applying the Glaze

After you have prepared your glaze and it is ready for use, you must decide upon a method for applying it.It is important to remember that when you glaze a piece you are coating the clay with powdered glass ingredients suspended in water.Color will appear only where it is applied.It is true that glazes flow out to even themselves as they melt and fuse to the clay body in the kiln, but they will not necessarily flow over a bare spot to hide it fully.

There are three main methods for apply­ing glaze to a ware: dipping, spraying and brushing.Dipping is generally used by the studio where a large number of decorated bisque items are to be glazed and fired.The glaze is generally purchased in bulk in dry form.Water is added according to the manufacturer's instructions.Once the proper weight of liquid glaze is ascertained, it should be checked before each day's use and water added if necessary.For example, it has been found that a mixture of liquid glaze weighing about 22 ounces per pint is approximately the right consistency for the average piece to be fired at tempera­tures between 2174 degrees and 2246 de­grees Fahrenheit.

-Glaze gum is sometimes added to toughen the unfired glaze to facilitate handling.The glaze you purchase may already contain gum.If you are not sure, ask your supplier whether gum has been added.If additional gum is subsequently added to an excess, drying is considerably retarded and much time is lost before the piece can be handled and fired.

Dipping processes are many and varied.Some pieces, such as figurines, are often glazed only on the outside.The piece is simply held at the base by the most con­venient grip and immersed headfirst in the glaze.It is advisable to have a dipping con­tainer large enough to permit a sweeping motion so that the piece can also be brought out headfirst, which causes the ex­cess glaze to drain toward the bottom of the piece.This will eliminate drops or bumps of glaze on the upper part of the figurine, which ordinarily would require smoothing before firing.The spots where the piece was gripped are then touched up with a brush.

Vases, cups, bowls, etc., which require glaze inside and out call for a slightly dif­ferent method.Some ceramists prefer to pour glaze on the inside, filling the piece and emptying it in rapid motion.This is allowed to dry and then the outside is dipped to the top edge to complete coverage.

Others prefer to accomplish the glaz­ing in one operation.By placing your thumb on the top lip of the object and one or two fingers on the base, the piece may be immersed in a sweeping motion, making sure the entire inside surface contacts the glaze, and brought out in a reverse motion.As in all processes, practice will determine the best-suited method for you.

Although there are inexpensive spray guns obtainable on today's market, a minority of hobbyists have the necessary equipment for glazing by this method.A spray booth, homemade or otherwise, is required so that the excess glaze may be recovered.A protective respirator should be worn since the glaze spray is readily in­haled and can be harmful to the lungs.

Even coverage of the piece and proper thickness of the applied glaze is not as easily controlled as it is by dipping.The glaze should be thinner in consistency so that it will readily pass through the spray-gun nozzle.To learn how to glaze with a spray gun, it is recommended that you get first-hand instruction.It is almost a "must" where spraying is concerned.Some hobby­ists have utilized hand-pumped insecticide guns for this glazing purpose.Try it if you wish, but work out-of-doors in order to disperse the harmful fumes.The glaze solution should be as thin as milk.Pour it into the barrel of the insecticide gun and spray the entire piece with at least three coats.Since there are so many variables involved in any form of spray-glazing, this method is not recommended for tyros.

Applying glaze with a brush is generally used with colored glazes where unusual decorative results are desired.

The recent general trend has been toward the use of one-fire glazes.These can be bought in liquid, ready-to-use form at most supply houses.The term "one-fire" ap­plies to the process of putting glaze on the unfired clay piece, letting it dry, and then firing only once.Brushing has thus come into prominence as a glazing method for the hobbyist.Beautifully decorated objects can be produced with brushes and one-fire glazes without requiring an extraordinary artistic talent.

The method of brushing glaze is easy.The first coat should be "drawn out," that is, actually brushed out—much like apply­ing a primer coat when painting a house.A minimum of two coats is usually re­quired.The second and successive coats are "laid on." This means the brush should be well filled with the prepared glaze, which is permitted to flow onto the piece without excessive pressure on the brush.

The number of coats which should be ap­plied depends on the depth of the color de­sired.Also, the individual's idea of a coat must be considered.One person's three-coat coverage might well be as thick as another's five-coat application.Many beautiful and unusual effects can be ob­tained by using various color combinations side by side which may flow together, and by brushing one color over another.

With the typical piece of glazed ware, you should not glaze the underside.At least it should have an unglazed footing.If the footing has been covered with glaze, which is probable if it has been dipped, you can remove the glaze with a piece of fine sand­paper after the glaze has dried, but before it has been fired.This process is called dry-footing.

Once you've gotten into glazing you'll find that orderliness helps.Store glazes on shelves, sep­arating flowing, crackle, underglaze, matte, etc.

Photographs on this page illustrate a variety of kiln firing errors.In the pitcher photo you see a mottled appearance made by bubbling during the firing; cause was too-thick application of glaze.Elephant above was correctly glazed in red but was overtired, thus causing the red to fire out in some places; it's a beautiful piece that everyone wants to make, but the question is: how much was it overfired? Vase is marred due to using flowing instead of regular glaze; painted part became covered.

Glazes, in and of themselves, their choice and application, involve a complicated chemical change.There are many varia­tions and their behavior is sometimes er­ratic, although much of the guesswork has been removed for hobbyists in recent years by ceramic engineers.The best advice is to restrain your expectations until you see the results.Constantly experiment and keep careful records of the experiments.Be alert to new and different qualities as they appear.

Glaze Defects

There are several common and recur­ring defects which you should recognize in glazes and be able to correct.These in­clude crazing, crawling, flowing, peeling.

Crazing is the appearance of small cracks throughout the glaze.It is caused by the glaze contracting more than the clay in cooling.This is the main reason for com­bining only glazes and clays which have been designed for each other.Crazing ef­fects are not desirable for utilitarian pot­tery because water and food juices would seep through the cracks.

Important for the ceramist who wishes to experi­ment seriously with qlazes is the ball mill, a device that revolves and causes porcelain jars containing powdered glazes and Hint pebbles to turn continuously, thus thoroughly mixing glaze.

However, there is a decorative technique, called crackling, by which the glaze is crazed purposefully by the ceramist.In fact, oil paint or India ink is often rubbed into the surface to accentuate the cracks.This type of glazing is reserved for sculp­ture or such objects as cigarette boxes, banks, etc., which need not be impervious to water or food.

A glaze is said to "crawl" when it pulls away from the body in small areas and ex­poses the clay.Refiring to a higher tem­perature, or applying a second coat of glaze with the addition of a flux, such as feld­spar, and firing again will sometimes remedy crawling.

If a glaze is fired beyond the tempera­ture for which it was intended, it may flow off the piece.Low percentages of flint or china clay added to the glaze will stiffen it and prevent flowing.But it is much wiser to fire a glaze precisely at the right tem­perature and not take the gamble of com­pensating for error.

Either the glaze, or possibly the glaze and decorative slip, may peel or break" from the surface of the piece.One cause may be a too-thick application of slip, or applying slip on ware that is too dry.•

Decals that can be applied over or under some glazes are available.Shown here is strip of decals behind a piece to which some are already added.

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