Making glazes from the raw ingredients is fun and highly rewarding, and leads to a more individual pot than if a commercial glaze were used. However, many potters are put off by the apparent complexity of the subject and by the myriad of materials involved. In this article, the raw materials used in stoneware glazes (above about 1200C or cone 6) are discussed in a novel and simplistic fashion. Designing a glaze does not necessarily require a greater technical ability than that needed to bake a cake!

The golden rule is to keep things simple. A glaze can be understood as a combination of three materials:
Silica – gives hardness and glassy quality to the glaze.
Alimina – strengthens the glaze, helps it to bond to the clay body, and increases the glaze viscosity so that it doesn’t run off the pot.
Fluxes – required to lower the melting temperature of the glaze mix. They include the alkaline elements potassium, sodium and lithium, the alkaline earth elements calcium, magnesium and barium, and also many of the colorants such as iron (particularly in reduction), cobalt and copper, which are usually insignificant due to the small amounts used.

For simplicity, only 3 fluxes are recommended for general use:
Calcium Oxide. (Sources: whiting, dolomite, wollastonite, calcium feldspar.) The most important stoneware flux, an alkaline earth oxide that can have a bleaching effect upon the glaze.
Magnesium Oxide. (Sources: dolomite, talc.) Gives a creamy finish to the glaze, and inhibits crazing. It may result in mattness and colour changes at high concentrations or in cooler areas of the kiln.
Potassium Oxide. (Sources: feldspars, neptheline syenite, cornish stone.) All alkaline oxides give a similar and good colour response, but used as the only flux they give brittle, crazed glazes. Potassium is chosen over sodium since it doesn’t volatilise (burn off), which is especially important if you have expensive electric kiln elements. It is also slightly less prone to crazing and attack by acids. Some people argue that sodium gives a better colour response, although the Chinese often preferred to use potassium in their copper red glazes! The other alkaline flux used by potters is lithium, which is relatively expensive, but has a far lower thermal expansivity, and consequently small percentages may be added to reduce crazing in high alkaline glazes.

Other useful fluxes include barium, which can enhance both colour and texture, but is very toxic. Its high melting point inhibits its full dissolution into the glaze melt, and since it is slightly soluble it should not be used on functional surfaces, especially if the glaze is low in silica. Boron and zinc oxide may be used as additional fluxes at the lower ranges of stoneware, and may also enhance colour and inhibit crazing. Zinc boils off very toxically in reduction, and so is only used in oxidation.

There are many sources of each of these elements, with each source often containing more than one of them. It is unlikely that the composition of these raw materials is known accurately. Using more than one sources of a particular element will add uncertainty to the amount of that element used, and also unnecessarily increases the complexity of your glaze recipe. In addition, raw materials are never pure, each containing an array of trace elements, which may affect the glaze. By sticking to the same source, at least these are constant, and if these elements are specifically required they may always be added later.

For simplicity the list of ingredients will be reduced to 5 raw materials, which can supply the 5 necessary oxides:
China Clay – a source of alumina, but also contains silica.
Quartz – a source of silica, and is usually fairly pure.
Potassium Feldspar – a source of a potassium oxide (and usually some sodium too), and also some silica and alumina.
Whiting – a source of calcium oxide.
Dolomite – a source of magnesium oxide, but also supplies calcium oxide in roughly equal proportions. In the rare occasions that a greater fraction of magnesium oxide than calcium oxide is required, talc may be replace dolomite.

It is the oxide that is of interest to potters, since this is the form an element generally takes when it enters the glaze melt. The numbers shown in the table give a rough guide to the quantity of an element’s oxide in each of the raw materials. For those technically minded potters, these numbers are derived by taking the typical percentage analysis of the raw material, and dividing each of these parts by the molecular weight of the oxide in question (and multiplying by 10). Material that burns off, such as the carbonate bit or chemically fixed water, is of no interest, and its mass is accounted for here. For those who wish to avoid as much mathematics as possible, these numbers tell you roughly the quantitiy of an element’s oxide is each of the raw materials, i.e. the oxide content.

ELEMENT	RAW MATERIAL	OXIDE CONTENT
alumina	china clay	silica 8, alumina 4
silica	quartz	silica 16
potassium	potassium feldspar	potasium oxide 1.5, silica 11, alumina 2
calcium	whiting	calcium oxide 10
magnesium	dolomite	calcium oxide 5, magnsium oxide 5
magnesium	talc	magnesium oxide 7.5, silica 10

For those Ludites who insists on measuring out their glazes by volume, rather than weight the following table is relevant. It is less accurate to work this way, and quantities will vary depending upon factors such as grain size, compactness and humidity. However many potters have successfully worked this way for many years, and it is without doubt less work.

ELEMENT	RAW MATERIAL	OXIDE CONTENT BY VOLUME (aprox)
alumina	china clay	silica 4, alumina 2
silica	quartz	silica 12
potassium	potassium feldspar	potasium oxide 1, silica 8, alumina 1.25
calcium	whiting	calcium oxide 8
magnesium	dolomite	calcium oxide 5, magnsium oxide 5
magnesium	talc	magnesium oxide 5, silica 6

Note in this article only oxide content be weight is considered.

EXAMPLES of how to use these numbers:
1 part of quartz added to a glaze recipe, adds about the same amount of silica as adding 2 parts of china clay. 1 part of china clay adds roughly the same amount of alumina as 2 parts of feldspar.
An example of a simple stoneware glaze recipe for a clear glaze at cone 10:

Feldspar	2
Whiting	1
Quartz	1
China clay	1

This glaze has a potassium oxide content of 2 x 1.5=3, and a calcium oxide content of 1 x 10=10, so although more feldspar than whiting appears in the recipe, calcium is the major flux. Interestingly, more silica in this glaze is supplied by the feldspar (2 x 11=22) than the quartz(1 x 16=16).

Finding ratios between oxides is also easy. For example the aluminium (total 2×2 + 1×4 = 8) to silica (total 2×11 +1×16 + 1×8 = 46) ratio is 8:46 or 1:5.75.

There are only 5 ingredients to get your head around, no more complex than in baking a cake. Although the white powders look similar, you must learn the difference between flour, sugar and salt if you want something good out of the oven.

An easy way to start is to take an existing glaze recipe, and to simplify it to include only the 5 ingredients here. Don’t bother with a calculator, round all quantities to the nearest 10%. Perhaps start with the above glaze and a few variations, such as 10% or 20% additions of each of the ingredients. Try your glazes on small test pots. Use the results of the firing to guess at what an improved glaze recipe may be, and then test again. Finally, once you’ve found a glaze you like, you can start experimenting with additions of colouring oxides and opacifiers, and the fun continues.

The following table gives the analysis of commonly used materials. The percentages of oxides entering fusion have been taken from Hamer, The Potters Dictionary, appendix table 30

	SiO	Al2O3	CaO	MgO	K2O	Na2O
WHITING			10
DOLOMITE			5.5	5.2
TALK	9.5		0.4	7.7
WOLLASTONITE	8.3		9.4
QUARTZ	16
CHINA CLAY	7.8	3.8
BALL CLAY	7.8-12	3.2-1.9	0.2
POTASSIUM FELDSPAR	11	1.8	0.2		1.6-1.1	0-0.3
SODIUM FELDSPAR	11	1.9			0.3-0.1	1.4-1.9
CALCIUM FELDSPAR	7.1	3.6	3.6
BASALT	8	1.4	2.5	0.3
CORNISH STONE	13	2.4			0.4	0.6
NEPTHELINE SYENITE	9.2	6.0			0.9	1.3
BENTONITE	8.5	1.9	0.3	0.8
SODA ASH						8.5
POTASH					7.2