Rules of Thumb for Choosing a Flux Set

This method of glaze research involves systematically varying the three main variables in a base glaze, i.e. fluxes, alumina and silica. By this method a given set of 35 glazes has one set of fluxes that doesn't change, and varing amounts of kaolin (the source of varying alumina) and silica.

So our first task in designing a set is to choose the set of fluxes and the relative proportions of each. We do this by choosing one or more "flux materials" from the Flux Materials List and deciding the relative proportions of each.

For example we might choose feldspar, whiting and talc in the proportions 3 to 2 to 1:

If we feel confident to choose a set of fluxes, then we go to the Calculation Page.

Otherwise read on.

Some Simple Rules of Thumb

We are choosing a set of "flux materials" and the proportions of each. This choice actually becomes Glaze No.31 (Glaze C) in the set, and it is the starting point for the whole set.

1. Among the flux materials, we usually choose at least one that contains alumina and silica as well as the flux component. In the Flux Materials List these materials are marked with an asterisk *. These are materials like feldspars, frits, and many wood ashes, powdered rock materials etc. We can use these materials up to 100% in Glaze C, but we usually add other fluxes as well.

2. Unless we have very specific objectives, we can put together almost any mix of fluxes in almost any proportions and expect to find glazes that are useful or excellent under some conditions of firing and clay body. There is no "correct" temperature or firing cycle for a given set. In a particular firing, there will always be some glazes in a set that will be useless (e.g. over- or under-fired) but the systematic variation of kaolin and silica determines that useful (and regularly excellent!) glazes will appear at just the right combination of kaolin and silica. Usually we don't know just what this combination will be until we do the experiment. Any set can usually be usefully fired over a range 200 deg C. or more.

3. To get most of the 35 glazes to melt at mid fire (1200 Deg C.), we can put up to 35% of frit into Glaze C.

To get most to melt at earthenware (1100 deg C.) we can put 50%, even up to 100%, of frit into Glaze C. (It is possible to achieve low temperature glazes with just the right mix of "stoneware" fluxes, alumina and silica, but most of our 35 glazes in the set will be unfused.)

To choose a set of fluxes to achieve a specific purpose requires some knowledge. The method presented here enables the beginner to achieve a lot with little or no prior experience, and in so doing actually begin to acquire that knowledge.

For the student who understands little about glazes, what follows is a number of sets that can get them started. What is given is "Glaze C" for each set. If we choose one of these and input it to the Calculation Page, the 35 recipes for the set can be read off or printed out, and also the batch recipes for the 4 "Corner Glazes" if using volumetric blending to prepare them.

Example Sets

Glaze C Recipes

The best temperature range for the set is indicated by (E,M,S) = (Earthenware, Midfire, Stoneware)

Set 1 - Limestone Set - (S)

70 Potash Feldspar
30 Whiting

Set 2 - Feldspar Set - (S)

90 Potash Feldspar
10 Whiting

Set 3 - High Lime Set - (S)

30 Potash Feldspar
70 Whiting

Set 4 - Magnesia Set - (S)

60 Potash Feldspar
17 Whiting
23 Heavy Magnesium Carbonate

Set 5 - Zinc Set (Oxidation only) - (M,S)

60 Potash Feldspar
17 Whiting
23 Zinc Oxide

Set 6 - Barium Set - (M,S)

45 Potash Feldspar
15 Whiting
40 Barium Carbonate

Set 7 - Low Temperature Set (Oxidation only) - (E,M)

60 Ferro Frit 4108 (also 4508 or 3134)
20 Nepheline Syenite
20 Zinc Oxide

Set 8 - for Nickel Pinks and Blues (Oxidation only) - (M,S)

40 Potash Feldspar
40 Barium Carbonate
20 Zinc Oxide
(suggest adding 2% nickel oxide)

Set 9 - Talc Set - (M,S)

30 Ferro Frit 4108 (also 4508 or 3134)
30 Nepheline syenite
20 Whiting
20 Talc
(suggest adding 0.5% cobalt carbonate)

Set 10 - Alkaline Set (Includes some matt alkaline glazes) - (M,S)

50 Barium carbonate
30 Ferro Frit 4110 (or 3110)
20 Whiting
(suggest adding some colourant)

Set 11 - Earthenware Alkaline Set - (E,M)

100 Ferro Frit 4110 (or 3110)
(suggest adding some colourant, e.g. 2% copper carbonate for turquoise)

Flux Characteristics Chart

The chart below gives a summary of some properties of the main fluxes. By studying this we can see how to choose fluxes to promote for example alkaline colour response, or adjust glaze fit. This is not a complete list by any means, and the student could get more information from references like Frank Hamer's "The Potter's Dictionary of Materials and Techniques" (Watson Guptill) and also "Stoneware Glazes: A Systematic Approach".

[Note: The reference to "Reduction Only" in relation to Iron Oxide in the chart below refers only to it's role as a flux. As a colourant, it can be used in either oxidation or reduction]

We can choose a safe set of fluxes if we wish or alternatively choose fluxes at random, probably biasing towards high temperature fluxes if firing high, or vice versa if firing earthenware. We should not be afraid to take big risks, especially if we have a group helping and doing lots of sets. It's quite unusual to get a set that doesn't do something interesting in the right conditions on the right clay, and the more risk we take the greater the possibility of finding something exciting and new. We should push the boundaries, explore the fringes. But we mustn't lose sight of our responsibility to use glazes appropriate and safe for whatever use we choose. Remember there is no "right" firing cycle for a given set. A set that seems boring at first might produce brilliant things when we change firing conditions or clay body. The aim is to reveal exciting new glazes and discover important basic glaze principles. The new glazes seem to fall out of almost every set. The glaze principles become apparent as a result of the orderly way the experiment is designed.