Testing Stains in slips and glazes
Protect your lungs! Anytime you are working with dry materials, you should wear a properly rated and well-fitting respirator!
We're going to explore this process mainly through an exercise of testing one stain color in various concentrations in a decorative slip. This will provide a basic understanding of the process, which you can then build upon with other testing methods.
Step 1: Establish Your Base
To begin, we need to identify a base slip formula. There are thousands of formulas out in the world, in books and online, so do some hunting and choose one that has a good amount of community feedback. You will want to find a white or off-white slip that is intended for your firing range. For surface slips, you will need to consider the clay body onto which the slip will be applied. Choose a stoneware slip to apply to a stoneware body, a porcelain slip to apply to a porcelain body, etc. This will help match up the shrinkage rates of the two bodies and help them fit together better, minimizing cracking. Your base recipe should add up to 100, and the colorant (Mason Stain) will be in addition to that. It's a good idea to test a few different slip formulas on your clay body before committing to one and moving forward to the next step. If all else fails, you can save your clay scraps and let them dry out. Pulverize them into a powder to get your dry slip.
It's tempting to go hog-wild and want to test every color at once, but do yourself a favor and just pick one. You can come back and repeat this process again and again. Sticking with one color at a time helps avoid mistakes, especially when you are first starting out.
Step 3: Prepare Test Batches
Decide how many different increments of the Mason Stain you want to test. I'd recommend at least 5 to give yourself a good amount of data and allow for a wider range. For the sake of this exercise, we'll stick to 5. We need to make up 5 separate mini-batches of our chosen base slip. We don't need much for each test, so 100 grams should be plenty. We will be working at a very small scale, so precision is crucial. Make sure you have an accurate gram scale on hand. Measure out your 5 batches of dry slip in 5 separate containers.
Now that you have 5 containers of dry slip prepared, it's time to add the colorant. Decide ahead of time what percentages you are testing and mark your containers and your test tiles. For this exercise, we're going to test the following percentages: 2%, 4%, 6%, 8%, and 10%. This is a reasonable range for most Mason Stains, however there are some that require higher percentages. Carefully measure your stain of choice and add them to your base batches. If you have 100 gram batches, this means you would add 2 grams of stain for 2%, 4 grams of stain for 4%, and so on. If you are mixing larger batches, calculate the appropriate grams for each batch. Carefully stir each batch to combine the dry ingredients.
Step 5: Slaking and Mixing
Add just enough hot water to each batch to completely cover the dry materials. Wait a few minutes for the water to make it's way down through the dry powder, stir gently, then add more water if needed to cover the dry ingredients again. It's best to cover your batches and let them slake for at least a few hours, if not overnight. Slaking is a soaking process that gives the water sufficient time to fully saturate the dry materials. Because of the extremely fine particle sizes, this can take several hours. Using hot water rather than cold water will help expedite this process. Once the batches are properly slaked, you can mix them up. I recommend an electric hand mixer or Jiffy Mixer. You may need to add a bit more water to achieve the correct consistency (heavy cream). Blend until smooth, then sieve with a 60 mesh screen to break up any remaining clumps of dry material. Be diligent to not cross-contaminate your test batches. Clean your mixing utensils and sieve thoroughly between each batch.
Step 6: Test Tiles
Hopefully you have planned ahead and already have some test tiles made up. If not, don't sweat it, just roll out a slab of your chosen clay body and cut out some small tiles. Make sure to stamp or scratch the percentages into the tile somewhere so you know what you're looking at when it emerges from the kiln. Apply 1-2 coats of each batch to it's matching (and well-marked) test tile, and let dry. Bisque fire, apply clear glaze to half the tile, and re-fire to maturity.
From these results, I can see that this particular red requires at least 10% saturation for a convincing red in a cone 5 slip. My next step would be to try a few higher percentages to see what deeper shades I can get, as the 10% is still a little too pink for my taste. Maybe I can get that fire-engine red at 12% or 15%. I'll just have to test and find out!
The process for testing Mason Stains in glaze follows the same procedures as previously discussed. Instead of dry slip batches, you will have dry glaze batches to which you will add your stain. While the initial process may be the same, there is much more room for error when formulating glazes. The chemical interactions in glaze are much more volatile than in slip, so there is a decent possibility you will run into some glaze defects along the road. Working through these issues by adjusting your recipe will take time, but you will be rewarded with a custom glaze that is all your own. Additional reading is highly recommended when pursuing glaze formulation.
John Britt article in Ceramic Arts Daily (photo below from this article)
DigitalFire article on formulating and trouble-shooting