It’s important to wear a NIOSH certified mask whenever using dry glaze materials.
I guess mixing up glazes isn’t that big of a deal, but I’m sharing my technique just in case there are some absolute beginners out there.
I find it easier to use a digital scale, see my article here.
Glazes “don’t travel well”, in other words materials, application, and firings vary from studio to studio. Even for well-known glazes, it’s important to first make a small tests. For these tests, I use 50g or 100g of material and apply the test glaze to a number of different clay bodies.
I use the Glazy Batch Calculator on my phone which will show you the subtotals for arbitrary amounts of total glaze materials.
Once I’m happy with a test, I mix up a larger batch of 1-2Kg. 1Kg is enough material to glaze small cups, 2Kg is a good amount for small bowls. These larger tests should reveal any problems with glaze suspension (is bentonite required?), application (cracking, peeling, etc.), and fired glaze defects. Once you have some nice results with 1Kg, you can finally move on to a big bucket of 5-10Kg.
Mixing up a test
Flat test tiles require the least amount of glaze for application. Here’s my article about how I make test tiles.
Sieve Mesh Size
For “natural” glazes containing large-grained materials or ashes, or in cases where homogeneity is not a concern, it’s fine to use a larger screen of 60-80 mesh. But in all other cases I use 120 mesh or smaller. Small mesh size is very important for glazes that contain small amounts of very important materials such as coloring oxides (e.g. cobalt and iron). But it’s also important to ensure that materials are adequately broken up and mixed (such as clays).
Below you can see two tests of the same batch of glaze fired in the same kiln. The glaze on the left was applied after passing the materials three times through an 80 mesh screen. The glaze on the right is the result of passing that same glaze once more through a 120 mesh screen.
Poorly dispersed colorants like iron are easy to see in fired glazes. But keep in mind that other “invisible” glaze ingredients like clays, feldspar, etc. also need to be well-dispersed and mixed in order to ensure the glaze melts properly. If you use a 60-mesh screen for tests and then a 120-mesh screen for large glaze batches, there will be differences between the fired results.
I’ve seen a few techniques for seeing into the kiln at high temperature. An old friend of mine still prefers blowing into the peephole, unfortunately on more than one occasion it has resulted in the particles resting in the peephole to be blown in as well, settling on the ware. The Jingdezhen firing masters I’ve met just put on an old pair of sunglasses and squint (on the rare occasions they actually need to look at a cone).
I’m currently using #5 welding goggles, the only pair I could find for sale here but they work really well. If you have a choice, go for IR rated lenses which protect from harmful infrared light. Here’s a really good article about eyeware for potters.
Combined with the goggles, a strong flashlight will give you a really good view inside the kiln. This year, my old LED flashlight finally gave out, and at around 400 lumens it was still a little difficult to see in the kiln. The LED flashlight I purchased as a replacement was on sale for about $40USD, a little expensive but to be honest I just wanted to know what 2000 lumens would look like. It’s blinding! But using this flashlight I can see all the way to the back of the kiln even in reduction at 1300° C (my kiln is only 1 meter long). You can even see glazes start to glisten in the light of the flashlight as they begin to melt..
So if you’re getting a new flashlight for the kiln, I think you should go for at least 1000 lumens.