Balance

Now that we have a list of possible ingredients and a rough idea about their behaviour the next step is to combine them
in certain ratios. This is where the idea of balance in a glaze comes from. On one side of the scales is the glass forming
oxides and on the other side are the fluxes, in the middle are the stabilisers.

In practice the glass forming oxide is always Silica and Alumina is the major stabiliser used. So the real work of glaze making is the use of various fluxes and their proportion in the final mix. As you can see from the above lists, if we could deal directly with oxides alone it would be a relatively easy job to make up a glaze. But few of our materials are so pure, and those that are have been heavily processed and are more expensive. So we have to use the more common materials at hand.

Where do we get a glaze recipe from?

Sources include books, magazines, internet, workshops and, of course, fellow potters. However remember to check that the firing temperature of the glaze matches the firing temperature of your clay and kiln. Also start to analyse your glazes by using the above balance as a starting point. How much of the glaze is feldspar, silica or the other fluxes? This will help you compare glazes and discard glazes which fall too far out of your preferred range.

One habit to get into early on is to write everything down, even if it seems trivial now. A few months or years from now it will be hard to remember exactly what the glaze looked like. The notes at the very least should include the recipe, the temperature fired to and what the clay was.

You can also just make up a glaze recipe and if you follow the methods of testing that we will be covering a successful glaze should be possible at any temperature. As a general rule of thumb, when listing materials, we choose the most complex material first and work our way towards the simplest of materials. So a very common glaze recipe might start with Potash Feldspar, then list the clay, then the simple fluxes like Whiting and end with Silica.
e.g.: Leach's Limestone Glaze for Cone 8 (also known as the 4,3,2,1 glaze)
Potash Feldspar... 40
China Clay .......... 10
Whiting .............. 20
Silica ................ 30

Although the above example glaze recipe looks simple with only 4 materials used, in reality the oxides that will be present in the final glaze are as follows:

9.1:1 Si:Al Ratio

The numbers mentioned above are derived from the glaze recipe in a mathematical process that reduces the materials to their constituent parts, making comparisons between glazes that may have widely different materials possible. The procedure is called the Seger formula, or Molecular Unity Formula. It is not hard to work out, but it is a lot easier with glaze calculation programmes on computers (see page 5 for the details).

In many ways the art of glaze making is all about balances and ratios of ingredients. Much of the analysis of glazes is done by comparing the amounts of various oxides present in a glaze, both against each other and against known limits. The main ratios that inform us about a glaze are the silica:fluxes ratio, the alumina:silica ratio and the flux ratio.

Silica:Fluxes Ratio

This ratio is important because to make a glaze you need lots of silica, you also need to melt it with the fluxes. However, too much silica will mean that not all of it will be melted with the fluxes and often results in a matte surface. Too little silica and not enough glass has been made and again the surface will be matte. However, as a shiny glaze needs at least 60% silica and because it is an easy ingredient to source it is not a difficult ratio to correct.

Silica:Alumina Ratio

This ratio governs the degree of shine and how melted the glaze becomes. This ratio is expressed with the alumina part equalling 1. For instance a silica alumina ratio of 9:1 means 9 silica for every 1 alumina and also indicates that the glaze will probably be glossy. When the ratio is below 4:1 then the glaze will be matte.

Flux Ratio

It is in the balancing of the various fluxes that the real art of glaze making comes to the fore. With about 13 possible sources of fluxes a great many combinations are possible. As a general rule of thumb, the more fluxes in a glaze make it more stable and adaptable. So rather than using only one, aim for about three major fluxes. The fluxes can also be broken up into 2 main camps:

The other fluxes are quite individual and are:

General qualities: good as a supplementary flux, active from mid to high temperatures, small amounts help glaze fit and colour response, only used in oxidation.

General qualities: very useful low temperature flux, can be toxic in fired form if incorrectly used, low shrinkage, good colour response, only used in oxidation, soft and easily worn.

General qualities: a complex material, boric oxide can act like a flux, or stabiliser or glass former, generally used as a very early melting flux, useful over the whole range of temperatures, low shrinkage, good colour response, used mainly in frit form, good in both oxidation or reduction.

At this stage I should mention why I haven't talked about colour. That is because the colouring oxides are usually added in small percentages and have very little effect on the glaze other than colour. So it is important to first come up with a 'base glaze' which exhibits the qualities you want to see from a glaze, like the degree of shine or opacity.

These notes are very much a general introduction to the world of glaze chemistry. In summary the most important things to learn are your materials - what's in them, how do they react in the kiln and then you will gain confidence in combining them in glazes. Also keep in mind this idea of balance and ratios so that if a new glaze comes your way you can start to guess it's likely effects even before you fire up a test. Which brings me to the final point - remember to label and note down everything!