A glaze is a suspension of solids in water. Here are some parameters that are useful when discussing the glaze and its application.
Glaze coat thickness
The effect a glaze gives depends on how thickly it is applied. For some glazes the effect is dramatic and so it is important to quantify thickness rather than rely on vague terms such as ‘thin’ and ‘thick’.
The parameter I use is the mass of dry glaze per unit of surface area, expressed in grams per square meter. I use the symbol G to denote this.
Thus, G450 is shorthand for a glaze coating that has 450g of glaze (dry basis) for every square meter of surface. In rough terms, G450 is moderately thin, whereas anything over G1000 would be considered thick.
Solid loading
A glaze is applied as a suspension of solids in water. Another important parameter characterises the amount of solid in a given amount of glaze. Glaze density, sometimes called specific gravity or SG is one way of expressing this but a more useful parameter is the solid mass fraction. I give this the symbol F. It takes on values between 0 (all water) and 1 (all glaze i.e. powder).
An example
Suppose we are evaluating a new glaze or the effect a glaze gives on a specific type of clay body. We have thrown 3 identical straight sided cylinders on the wheel and now want to test the glaze at 3 different thicknesses, G400 (thin), G800 (moderate) and G1200 (very thick).
For illustration, I’ll use one of Derek Clarkson’s published crystalline glaze recipes, a 100g batch.
Component Amount [g]
3110 frit 46
china clay 4
silica 18
zinc oxide 24
titanium dioxide 8
+manganese dioxide 1.3
+cobalt oxide 0.5
+dispex 0.3
Water 32*
F0.76
The total solids here (ignoring the dispex) is 101.8g and the total mass of solids and water is 133.8g. The solid mass fraction, F is the solids divided by the total, in this case 0.76.
The amount of water used here is small by most glaze standards. The dispex (defloculant) is essential to being able to achieve a fluid mixture. Without it, the above would turn into a semi-solid that could not be mixed. With it, a low viscosity liquid is produced. F0.76 is an extremely high mass fraction for a glaze and the corresponding specific gravity will be around 1.90kg/l.
Moving on, we need to measure the dimensions of the cylinders after bisc firing. The glaze is to be applied to the outside vertical wall only so we need the overall height and the diameter.
For example:
height: 200mm
diameter: 75mm
We just use the formula for the surface area of a cylinder: π x 200 x 75.
This is 47123 mm2 or 0.047m2.
We can now calculate the amount of dry glaze needed for the three tests:
Cylinder 1, G400: 400gm-2 x 0.047m2 = 18.8g
Cylinder 2, G800: 800gm-2 x 0.047m2 = 37.6g
Cylinder 3, G1200: 1200gm-2 x 0.047m2 = 56.4g
The final step is to calculate the amount of fluid glaze needed. Here the parameter F is used, just divide by it to convert to mass of mixed glaze:
Cylinder 1, 18.8g / 0.76 = 24.7g
Cylinder 2, 37.6g / 0.76 = 49.5g
Cylinder 3, 56.4g / 0.76 = 74.2g
All that remains is to weigh out those amounts of mixed glaze and brush them evenly on the cylinders.
In this example, it would not be possible to brush the glaze evenly. If loaded onto a brush and applied to absorbent bisc ware, the glaze would dry almost immediately and the brush would probably stick to the pot. For a real example, this would be overcome by using a gum addition to the glaze (not more water). My article on glaze additives covers this.