Controlling Glaze Application Thickness on Porous Bisqueware.
Factors controlling the thickness of a glazecoat on bisque.
- Length of time in the glaze
- Density of the glaze suspension. That is how much water is there and how much suspended powder.
- (Apparent) porosity of the bisque, including how dry it is, how much pore space it has, how quick the pore space absorbs water, and how thick the bisque is.
- Rheologic properites.
- a. flocculation
- b. surface tension and viscosity
- c. number of long molecules (might be covered in viscosity)
- d. The amount of fine particles that can clog surface pores.
Length of time in the glaze
When you dip a piece in your glaze suspension the bisque ware starts to absorb water first making the glaze near the surface a more dense liquid and then turning it solid. So long as the bisque is absorbing water fast enough the glaze coat continues to thicken. As the absorption slows down there reaches a point where the coat of stiff glaze starts to get wetter again and slough off. The thicker the work is, the thicker the glaze can get and the faster it gets thick. In beginning thrown work the base of the pot is often thicker than the top making the glaze thicker near the bottom, just where running has the biggest likelihood of causing an issue.
Dipping the work in water before glazing decreases the availability of pore space for absorbing glaze. Right after you dip it the effect is greater. Because water without glaze absorbs quickly these have to be very fast dips. With work that is thicker near the bottom you can dip the bottom few inches in water before you glaze and if needed pour a little water on the inside bottom and pour it out. I do this with really runny ash glazes so that they will not run too thick on the inside.
How long a pot is in the glaze is perhaps the primary method of control of glaze coat thickness. If you imagine pushing a cylinder in for 5 seconds and then removing it for five seconds, the first part of the pot to enter the glaze will be in the glaze for ten sends and the last for less than a second. If you want an even coat of glaze, you will not have it. I use the words plunge, wait, pull. Don’t go so fast that you create a tidal wave or splash but do not take your time putting the pot in, or taking it out. After you pull it out you usually want to keep it in the same orientation so that you do not get drips down the side of the pot.
If you are doing two different glazes, the amount of time you wait between glazes controls the thickness of the overlap. The longer you wait, the drier the first glaze becomes and the more porousity it has avaialble to dry the second coat of glaze. Being ready with the secoond glaze saves loads of problems. As soon as the high sheen is gone it is usually safe to dip in the second glaze.
More solids in the glaze means that the pot has to absorb less water to make a stiff coat. This speeds up how quickly a coat accumulates. Adding water can work to a point but it also increases the shrinkage of the coat as it dries. With too much water sharp edges of the clay become saturated and get little or no glaze. There are many ways to test the thickness of a glaze coat and to control it. The first measure of control is the density. How much does a given volume weigh? Adjusting that by adding water (it decreases the density of the glaze) is the first thing to do after checking if it is too dense.
Glazes should be stirred immediately before glazing. Some glaze mixtures are particulary sensitive to this. Further, since materials settle out at different rates an unstirred glaze is a different glaze at the top than the bottom. There is a particular watery look to the last part of a pot dipped into an unstirred glaze.
The rheology of the glaze is the next issue to deal with. As you speed the absorption of the water needed to stiffen the coat and as you reduce the water needed to be absorbed you cut down on the space between the particles of glaze. At least this is the theory of Matt Katz, and it makes sense to me. This decreases the amount of air that will be trapped in the melted glaze coat and cut down on pinholes. Adding a deflocculant helps with this as it reduces the amount of water needed to make glaze fluid. Shorter dipping time also helps. Matt also favors low bisques because it increases the force and speed of water absorption decreasing the pore space in the glaze coat.
On the other hand flocculants seem to cut down the amount of thickness variation created by drips flowing off handles or bottoms of pots when they are pulled from the glaze slurry. Since you cannot deflocculate and flocculate at the same time, you have to do what is needed more depending on the glaze.
Fine particles, especially bentonite, also help to keep drips from setting in thick streams. The fine clays clog the surface pores as the pot is held in the glaze. So once the glaze reaches a certain thickness the rate at which it absorbs water slows down decreasing the impact of drips as you are applying glaze. It is a good reason to add bentonite to most any glaze. Veegum does this too. Glazes with lots of ball clay do not need the addition.
Other additives such as gums, glycols, can slow absorption even further. Some of these materials affect the rheology in multiple ways. They can be deflocculants, or flocculants, they can affect the surface tension or viscosity so test them. Make sure that your kiln is vented regardless and avoid things that you should not have your hands in or are hazardous to burn.
Ways to check glaze thickness
- Scratch through the applied glaze with a pin tool and look at the thickness of the coat.
- Look at the glaze coat and see how it covers details,rounds off rims, and look the thickness at the edge of the coat. This is harder than it seems and takes practice.
- Make a thickness gauge out of a dial indicator. I am hesitant to give directions as I have not used one.
- Make a thickness gauge out of a piece of metal with a series of teeth that will scratch into the glaze coat. I believe that I read about this in Cardew’s “Pioneer Pottery” but it could be Leach’s A Potter’s Book”
In order to do this you need some vocabulary, a mental scale of thicknesses. Although if you are using a dial indicator a numeric scale might make sense.
- Light Wash. A thickness where you see more clay than glaze. The wash is only thick in recesses if anywhere at all. Likely it does not show at all on sharp edges.
- Heavy Wash. The coat mostly covers the clay but you can see some clay showing through on flat areas of bisque. Usually it is thin on sharp edges.
- Just Opaque. A little heavier than heavy wash, you cannot see the clay on flat areas at all although edges may show.
- Photo Paper Thickness
- Half the thickness of a dime
- The thickness of a dime
- Nickel (US or Canadian coin)