POTTERY GLAZE FORMULA BASICS

The information below will help you:

• choose the right glaze for your work.

• alter glazes to suit your work.

• appreciate the subtleties of glazes and glazing techniques. 

• The Glaze Buddies (basic components of any glaze): Flux, Alumina, Silica and Modifiers.

Topics Here are:

• The Glaze Buddies (basic components of any glaze): Flux, Alumina, Silica and Modifiers.

• Glaze Science Terms

• Glaze Attributes

• The Effect of Fluxes

• Characteristics & Fluxes for the Temperature Ranges: Earthenware, Mid-range and Stoneware Glaze

• How to Make Glaze Tests on Test Tiles

• Math Tips for Glaze Making: Make a formula total 100%. How to estimate the amount of dry material to fill a bucket.

Online sources for more information:

• http://Matrix2000.co.nz/GlazeTeach

• http://digitalfire.com

The Main Components of Pottery Glaze

Glaze Science Terms

• Alkali—the opposite of acid, another term for flux. The stronger alkalis are lithia, soda, and potash; the weaker are called alkaline earths, and include magnesia, calcia, strontia, baria, zinc and lead oxide.

• Amphoteric—an intermediate oxide able to act as either an acid or alkali, which acts as a stablizing link between fluxes. Alumina is the most important; others common ones are boric oxide, red iron oxide, the opacifiesr titania, zirconia, and stannic oxide.

• Eutectic—the lowest point at which a mixture of 2 or more substances will melt, which is lower than the melting point of any one of the substances alone. The eutectic varies with each combination of materials.

• Flux—a glaze material that lowers the melting point and promotes fusion of different materials in a glaze.

• Frit—a combination of glaze materials that have been fired and ground down to powder again in order to

  • render soluble materials insoluble;

  • lower the fusion point;

  • lower the risk of coming into contact with caustic or toxic materials.

• Glaze—a combination of oxides fused by heat to form a glass—like coating on ceramics. It usually is a combination of Silica, Flux, and Alumina. (see The Glaze Buddies Al-X-Si)

• Opacifier—a mineral added to a glaze that makes it opaque, either by creating a structure of tiny crystals that deflects light, or remaining suspended and causing light interference, or causing minute bubbles that interfere with light passage.

• Oxide—a chemical combination (of atoms) of an element with oxygen.

• Reticulation—a glaze surface created during firing when one glaze bubbles up through the glaze over it to create rings of color, or a crater-like texture.

Glaze Attributes

• Alkaline Glazes—are characterized by bright turquoise blue from copper, pink from cobalt, and blue from iron, typical of early Egyptian and Persian earthenware pottery. The glaze is dominated by fluxes such as lithia, soda, potash, and baria, but low in amphoteric fluxes. Boric oxide is usually added to lower the rate of crazing of these brittle an unstable glazes that are not appropriate for food containers.

• Crystalline Glaze—large snowflake—like crystals can be encouraged to form by extremely careful control of the firing and cooling cycle. Reduce the alumina conten, use zinc oxide, Borax, sodium, potassium, rutile, or rion. Glazes run, so pieces should be fired on supports over saucers.

• Glossy —hard, reflective surface which may be opaque to clear.

• Mat / Matte—non-glossy finish due to the glaze composition. Barium (toxic) or alumina plus slow cooling help form mattes.

• Opaque—light can not pass through.

• Refractory—the quality of being resistant to the effects of high temperatures.

• Translucent—almost clear.

• Vitreous—the quality of being hard, glossy, and non-absorbent.

The Effects of Fluxes

Fluxes affect the melting point, color and texture of a glaze. Under each temperature range below are the:

• Characteristics of color, durablity, etc.

• The fluxes most often used in each temperature range.

• Most common types of glazes in that range.

Earthenware (cone 015-1)

• Characteristics

  • brilliant colors possible

  • often not durable enough for food serving

• Fluxes: Borax, Gerstley Borate, Colemanite, Lead, or Strontium.

• Some Common Types

  1. Luster—a thin film of metallic salts that are reduced either by the medium that holds them when being painted on, or introduced in the very low temperature firing. It is usually applied to an already glazed surface.

  2. Majolica (maiolica)—traditionally an opaque tin-based glaze with very colorful painted designs. It began on the island of Majorca in Spain, and then was adopted by Italy. Also called Faience for Italian city where it thrived.

  3. Overglaze / China Paint—ceramic enamel colors painted on a fired glaze surface and fired a second time (at a lower temperature than the first glaze firing, allowing bright red/orange that would burn out at a higher temperature.)

  4. Raku—a firing process that usually includes post-firing reduction to alter the soft glazes with an exaggerated crackle, metallic luster, or opalescent effect.

  5. Terra Sigillata—a very soft slip glaze originated by the ancient Greeks.

  6. Underglaze—oxides or commercial glazes or stains, painted under the glaze layer before firing.

Mid-range Glazes (cone 2–7)

• Characteristics

  • More brilliant colors possible than in the stoneware range

  • Colors similar to reduction effects may be obtained

  • Often durable enough for food serving

  • Economically fired in an electric kiln

• Fluxes: Some low-fire flux is substituted for high-fire fluxes.

• Benefits

  • Many potters use this range because it produces some of the benefits of the stoneware range, but also allows a wider range of color.

  • A second, and great, benefit is that many people find electric kilns more easily available and economical to use. Electric kilns normally do not fire above cone 6-8, and have an oxidation atmosphere. Even without the glaze effects possible with reduction in a gas kiln, the mid-range is an excellent alternative firing range.

• Books for More Information

  • The Complete Guide to Mid-Range Glazes: Glazing and Firing at Cones 4-7, by John Britt

  • Electric Kiln Ceramics: A Potter's Guide to Clay and Glazes, by Richard Zakin

Stoneware Glaze Types (cone 8-13)

• Characteristics:

  • subtle colors

  • can be durable enough for food serving

• Fluxes: potassium oxide, sodium oxide, lithium oxide, calcium oxide, magnesium oxide, barium oxide, zinc oxide, strontium oxide.

• Some common types:

• Celadon—a transluscent glaze first developed in China to imitate the color of jade, from the colorant iron.

• Chun—a pale blue opalescent glaze developed in China, made in low alumina wood ash and feldspar glazes with small percents of iron and copper.

• Copper Red(sang de bœuf, flambé)—an oxblood red created with a small percent of copper in certain reduction glazes.

• Crystalline Glaze—snowflake-like crystals can form in certain low alumina glazes with careful heating and cooling. Glazes often run and should be fired on stilts over a saucer.

• Salt or Soda Vapor Glaze—salt is introduced into a hot kiln causing it to vaporize. The sodium attaches to the silica in the unglazed clay body creating a glaze, often with an orange peel textured surface.

• Shino—an almost purely feldspathic white, pale iron red, gray, or marbelized glaze first produced in the Mino kilns in Japan, in 1580. It has a thick, very slightly transluscent surface that is desirable in Japanese tea ceremony bowls.

• Slip Glaze (vitreous engobe)—a glaze that contains a large percentage of clay. The advantage is that it may be applied to greenware rather than bisque to eliminate one firing.

• Tenmoku (temmoku)—a rich dark glossy iron-saturated glaze developed in China and Japan.

How to Make 5-10 Glaze Tests

1. Roll out a large slab no thinner than 3/8, up to 1/2 " thick.

2. Cut out 4 x 6" tiles.

3. Make a shallow groove about 1 1/2" up from the bottom, completely across the tile to catch the glaze if it runs during firing. The tiles should be glaze-fired vertically.

4. Leave a smooth area across the the upper part of the tile, and make a horizontal band of texture so that you see how the glaze looks on both types of surfaces. It may be useful to also add a band of white or dark slip.

5. Dry the tiles between layers of newspaper, and turn them over occasionally, to prevent them from curling up at the corners.

6. Bisque fire tiles and store them in a covered container to keep them clean.

7. Paint one line of each of the 3 main colorants (cobalt, copper and iron) mixed with water or commercial underglazes to test their reaction with the glaze.

8. Brush or dip glaze over the area above the shallow groove (one layer on left side, 2 layers on the right); wipe glaze off the edges and back of tile so that it can be fired vertically, leaning against a kiln post. It is useful to write the complete glaze formula and temperature (with iron oxide in water and a fine brush) on the back of each tile for future reference.

——————————————————

thin glaze . . . . . . . . . . . . thick glaze

___ Red Iron Oxide Line ___
___ Copper Carb. Line ___
___ Cobalt Carb. Line ____

-----------------------------------

texture band

-----------------------------------
optional band of white / dark slip

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To prepare test batch of glazes

• Wear an approved dust mask or respirator.

• Use one container with snap-on lid for each test.

• Label each container with its contents.

Prepare your clean bisque tiles with colorant lines.

Prepare the tiles by drawing one thin horizontal line in each of the 3 coloring oxide solutions*, across the tile just above the groove.

*2 recipes for oxide writing/decorating solution:

1. 1 tsp. colorant + 1 tsp. Gerstley Borate/Frit #3124.

2. 75% Frit #3124 + 25% colorant, + 1Tbsp liquid laundry starch.

Weigh out the ingredients in your recipe. 

TIP: Make multiple tests of the same base at the same time to more accurate and less time consuming. Line up 1 container for each test. Set the scale and weigh the same amount of the 1st ingredient into each container, then the 2nd ingredient, etc. Check off each amount in the formula as you weigh it, because the oxides look alike and you can easily loose track of what you are doing.

Math Tips for Glaze Making — How to Make a Glaze Batch total 100%

First, why would you want to do that?

Glaze recipes/formulas are often written so that the base ingredients total 100, which means the parts represent a percent of the whole. If all glazes are written this way, it is easier to compare the ingredients if you are looking for a particular effect from certain fluxes, etc.

If a glaze base does not add up to 100, below are 2 ways to change the numbers and still keep the ingredients in the correct proportion. If your math is correct, the new total should be 100.

EXAMPLE

OLD RECIPE RECALCULATED

Ingredient A - 85 72

Ingredient B - 16 14

Ingredient C - 12 10

Ingredient D - 5 4

TOTAL . . .  118 100 - New Total

The simplest way: Divide each ingredient amount by the total.

Ingredient A . . . . . 85 ÷ 118 = 72
Ingredient B . . . . . 16 ÷ 118 = 14. (13.5 rounded up)
Ingredient C . . . . . 12 ÷ 118 = 10.
Ingredient D . . . . . . .5 ÷ 118 = 4

If you like to use algebra:

For Ingredient A . . . . . . then repeat for each ingredient

85/118 = 1/100
118A = 85 x 100
118A = 8,500
8,500 ÷ 118 = A
72 = A

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How much dry material does it take to make a 5 gallon glaze bucket of liquid glaze?

Potters often use 5 gallon buckets to hold liquid glazes they make themselves. It's best if buckets are only 3/4 full to allow space for displacement when work is dipped into the glaze.

Here are useful facts to know.

1 pint = 1 pound "the world around"
1 gallon = 8 pounds
1 pound = 453.6 grams

How to figure:

5 gallons x 8 lbs per gal = 40 lbs

40 lbs x 3/4 full = 30 lbs liquid glaze needed.

About 1/2 the 30 lbs finished liquid glaze weight is water, so 
15 lbs dry glaze is needed.

15 lbs dry glaze x 453.6 grams per lb = 6,804 grams dry glaze

If you need about 6,800 grams of dry glaze materials,
you can multiply a recipe that adds up to 100 by 60 or 70 
to fill a 5 gal bucket 3/4 full.

About 6,800 grams of dry glaze materials makes a good amount for a 5 gal bucket.

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How would you compare costs of commercial glazes to those you make yourself?

1 average glaze batch is 6,800 grams / 15 pounds. Dry materials are ordered more economically in 25-40 pound bags. You will probably use the most common ones in different proportions in more than one glaze.