How does the thickness of your piece affect the glaze application?

In my free Strategic Glazing class, I talked about:

When the conditions are the same and we repeat our steps, then the result is going to be the same. 

Some of the conditions of our glazing process are:

• the water content, which we can control with specific gravity
• the viscosity of the glaze
• mixing your glaze really well
• the size of your piece
• the bisque temperature that you fire to

More pores = more glaze absorption

The reason why the size of your piece, or the thickness of your clay body will affect your glaze results (and this is related to your bisque temperature) is because the thicker your pieces, the more pores there are in the bisque for absorbing water during the glazing process. 

When we dip our bisque ware into the glaze, the water gets sucked into the pores of the bisque through a process called capillary suction. And that action, that suction motion, is what sticks the glaze particles to the bisque.

At some point, if you were to hold your bisque in the glaze and just hold it in there, at some point the bisque will become completely saturated with water. It won’t be able to absorb any more glaze particles. And then your glaze will essentially take forever to dry.

The thicker your bisque is, the more pores there are in the bisque, which means more water can be absorbed into the bisque, which means more glaze particles can stick to the bisque, giving a thicker glaze application.

So if you’re glazing very thin wares, then you get a much thinner glaze application before your bisque becomes saturated with water than if you were glazing thick pieces. That is why the thickness of your piece matters.

A lower bisque temp can increase porosity

For example, I work with translucent porcelain. I throw it on the wheel and I try to get the walls very, very thin. I was having problems with my bisque becoming saturated before I could get a good coat of glaze, and especially on the rims of my pieces. 

Since I didn’t want to thicken my pieces because I wanted the translucency of the thin porcelain, I lowered my bisque temperature. This is another way to increase the porosity of your bisque in order to get a thicker glaze application.

So that is the relationship between the bisque and glaze application thickness. The more pores that you have in your bisque (either by having a lower bisque temperature or by having just more clay) or having a thicker piece of clay, then more water can be absorbed and therefore, more glaze particles will stick to the bisque and that will cause a thicker glaze application.

And so for consistency, for the conditions to be the same, if you have 20 mugs that are all the same, you throw them all the same thickness, then it’s easy to achieve consistency because they’re all going to absorb the glaze in the same way.

But if your mugs are all very different, you have thick mugs and thin mugs, then dipping them all and holding them all in a glaze bucket for the same number of seconds could potentially give you different glaze results because the thickness of your glaze application will vary depending on the thickness and porosity of your bisque.

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