Best Practice Glaze Testing

A Quick Mix for Testing Multiple Colorants in a Base Glaze

By Jake Corboy

Artist Website:

Hello my name is Jake Corboy. I am formally trained in pastry/baking and I have found that the attention to details, and efficiency have been very useful in my ceramic projects. Follow me on Instagram @jakecorboy

One of the most helpful things I use from baking is efficiency, I am always thinking if there is a faster/easier way to do something while still retaining quality. For example to save time we would mix up a big batch of gelato base, then portion it out to add different flavorings like fruit purees, nut pastes or sauces. This allowed us to save a lot of time rather then if we weighed out each batch individually. From each master-batch we were able to create many delicious flavors while saving time and energy.

JCorboyGlaze1.JPG
JCorboyGlaze2.JPG

This method may also be applied to testing glaze colorants. Although with glaze it is a little different as you can’t taste how much colorant is in a glaze. Also in making a glaze there is a wet portion which is the water and the dry portion which will form the glass opon firing. You only calculate the colorant percent to the dry glass forming portion of glaze recipe. Weighing out the water while mixing up a batch of glaze for testing is very important. This allows the use of cross multiplication to figure out how much dry weight is in each smaller portioned out batch.

When testing colorants in a glaze, I mix up a big batch of the base glaze. I weigh out the same amount of water as the glaze batch. Add about 1/2 of it, then mix the glaze to a paste to help break up any stubborn lumps. I then add more of the weighed out water until the glaze reaches a sieveable consistency. I sieve the glaze then keep adding the weighed out water until I reach the specific gravity I want. If there is left over water, I weigh it and subtract it from the original water weight. If you need more water, weigh it before you add it to the glaze.

Once the large base batch is mixed and sieved to my liking, I can portion it out into smaller batches and test many colorant versions of the glaze. But in order to figure out how much colorant to add to each portion, I must figure out how much dry glass forming material is in each smaller wet portion. Since I weighed out the water I can use cross multiplication to accurately figure this out. In the example I mixed up a 3,000g batch of base glaze. I added 3,765g water to my base glaze. Any additive like bentonite must be accounted for as well and this recipe has 60g bentonite in it. So I have a total weight of 6,825g.

JCorboyGlaze3.JPG

I know how much dry glass forming material is in the big base batch (3,000g). I know the total weight of all the material in the base glaze (6,825g). I know the weight of the smaller portioned out glaze batches (100g). Now I just have to solve for X (which is the amount of dry glass forming material in the small portions). 

I solve for X using cross-multiplication.

3,000g X 100g / 6,825g = 43.96g   
X= 43.96g

X=43.96g. This is the weight I will use to calculate the correct percentage of colorant to add to each 100g wet batch. I am testing 5% additions of stains in this glaze.

So 43.96g x 5% = 2.198g stain added to each 100g wet batch. I use a scale that measures two decimal points for greater accuracy. Measuring out each stain at 2.18g - 2.20g. Weigh the colorants in a separate container then add them to the wet glaze.

To mix all these separate little containers of glaze/colorants thoroughly and quickly, I use a milk frother wand. This allows me to mix the colorants into each little batch in a matter of a few seconds, with no additional sieving necessary. The battery powered milk frother is fine if your only going to test a few batches every so often. But if you are really into colorant / glaze testing I recommend cutting off the frother part and using it in a Dremel. I use the lowest setting on the Dremel, around 5,000 rpm, any more than this and the frother head will start to wobble and your glaze will go flying out of the container. NOTE! These milk frother wands are not meant for these kinds of rpms, it works but be safe and aware of the tool. Also if you're going to use a Dremel, make sure you get a milk frother that has the “squirrel cage” type frothing head(like in pic 4). It is centered and doesn't vibrate at the higher rpms like the other kinds do.

JCorboyGlaze4.JPG

Now I have a bunch of colored glaze to play with. I can test them individually and still have enough left for color line blends or over under tests with other glaze bases. I use the leftover base for line blending with some of the darker colors and for mixing up more or larger colorant versions of the glaze. This is a very quick way to test multiple colorant versions of a base glaze. It has proven to be very accurate, repeatable and efficient.

Jake Corboy is from Seattle,Wa and has been working with clay for 20+ years. With a background in baking/pastry and residential remodeling he has utilized skills and techniques learned in these industries and applied them to his ceramics.  He spent the 2017-8 year working at Rat City Studios as a studio assistant.  He makes multi tiered hand textured wheel thrown pottery and slip cast sea creatures. He also enjoys testing copious amounts of different glaze combinations using the above method. He has just started utilizing this method and posts his results on his website.