Jake Corboy | Ceramics & Pottery | Seattle WA.

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Testing Colorants

This method is a fast way to test many colorants in a base glaze. Once I have found a base glaze that I like. I will make a big batch of it and portion it out for colorant testing and mixing.

There are generally two portions to a glaze: wet and dry. The wet portion is the water which acts as a carrier for the dry portion that forms the glassy surface upon firing. When calculating the percent of colorant to add to a glaze, you use the weight of only the dry glass-forming portion of the glaze. If colorant is calculated using the weight of both the wet and dry components together, the color will be way too saturated.

 

When testing colorants in a glaze, I begin by mixing up a big batch of the base glaze without any colorants. I weigh out the same amount of water as the glaze batch, as an initial starting point. I add about 1/2 of it to the dry portion, mixing the glaze while its kinda thick 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 any leftover water, I weigh it and subtract it from the original water weight. If more water is needed, I weigh it before adding it to the glaze batch.

When the large master batch of glaze is mixed and sieved to my liking, I then portion it out into smaller batches and go to town on testing many colorant versions of the glaze. How do I figure out how much colorant to add to each portion? I must determine 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 master batch of dry glaze material and added 3,765g of water to it. Any additive like bentonite must be accounted for as well and this recipe has 60g bentonite in it. In the end, I have a total weight of 6,825g of sieved glaze slurry.

Once I have determined how much dry glass-forming material is in the master batch (3,000g), the total weight of all of its’ ingredients -wet and dry- (6,825g), and the weight of the smaller portioned-out glaze batches (100g),  I just have to use cross multiplication to solve for X (which is the amount of dry glass-forming material in the smaller portions).

3,000g Dry glass-forming material in the master batch

6,825g Total weight of everything in the master batch

X amount of dry glass- forming material in wet portion

100g Portioned-out wet glaze

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 of 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. I weigh the colorants in a separate container then add them to the wet glaze.

In a matter of a few seconds, colorants can be mixed into each portioned-out container of glaze with a milk frother wand, with no additional sieving necessary. A battery-powered milk frother works fine for testing the occasional batch of glaze. I have also seen mini paint mixers on airbrush websites which would probably work too.

This method will yield a lot of colored glaze to play with. The colored glaze can be tested individually with enough left over for color line blends or over under tests; it is a very quick way to test multiple colorant versions of a base glaze. It has proven to be very accurate, repeatable and efficient.