Water content in clear glazes

If you don't like your clear glaze surface quality, you might need more water than you think. I did some testing to find out how much water is too much.

Backstory: why do I have 2 clear glazes

One day I quit my community studio and got very excited about converting my basement into my own studio. Getting my own wheel, my own kiln, my own signature glazes! The world was my oyster. I felt very encouraged by all the blog posts claiming how “easy” and cheap it is to mix up your own glaze (have you ever baked? then you can mix a glaze too!). I picked up John Britt’s book “The Complete Guide To Mid-Range Glazes” and set out to find the best clear glaze… which was a disaster. I think all the “learning opportunities” that I had while trying to make a glaze is a good topic for another blog post in the future. Only months later did I hear that making a clear glaze is one of the hardest tasks. Anyway, I finally found success with Tony Hansen’s G2926B cone 6 clear glaze (works great in cone 5 as well). It’s the most clear, smooth, and stable (underglaze stays put) glaze I have ever made. Until I bought a bag of NZ6 porcelain from Clay Art Center and realized that every glaze I know of crazes on that clay body, and I had to start searching for a clear glaze from scratch. Eventually, Clay Art Center staff pointed me to Laguna Clear Bright MSD-29 (which they stock but don't have a listing in the online store), and I settled on a commercial glaze. I don’t intend to use more of NZ6, I just wanted to finish the bags I already bought and I wasn’t in the mood to reinvent the wheel. Thus, currently I have these 2 clear glazes in my studio: Clear Bright for NZ6 and G2926B for the EG6 porcelain that I intend to use for the foreseeable future.

Problems

These 2 glazes were fine for just plain white pots or small underglaze decoration. However, I also started playing with sgrafitto and larger colored surfaces, and I quickly noticed that my pots don’t look nice. G2926B was getting cloudy in some areas, and Bright Clear had lots of not-so-small clearly visible bubbles. Here is a closeup of Bright Clear (left/first) and G2926B (second/right):

 

I was confused. I thought I had followed all the instructions and tips on how to mix the glazes. I was dipping them for a short amount of time, literally in & out of the bucket. I felt I needed to go thinner on the application but I was scared of thinning the glaze itself - what if I add too much water and the glaze is ruined? Also, why don’t I see any discussion or help articles about how much water is too much?

Problem solving

I figured I need to do a hands-on research (not just reading). I did some “line blends” by gradually adding water to small samples of my glazes. In the first series of tests, I took 200ml out of each clear glaze bucket and added 15ml water (7.5% by volume, 5.5% by weight) 4 times. I noticed that each time I dipped my test tile, the glaze layer was thinner, which I also confirmed by weighing how much glaze each tile absorbed. The 4th tile seemed to have a really thin layer of glaze, but when I fired the tiles, all looked good. Surprisingly, the 4th tiles looked the best! I was nowhere near the water limits in my glazes.

Encouraged by the results, I added some water to my big glaze buckets, but I was too scared to add 30% water by volume (4*15ml/200ml). I set out to find the water limit. How far am I from ruining my glaze with water? In the second series of tests, I took out 150ml of each clear glaze (which were already thinned a bit after previous testing) and added 20ml water 4 times. I finally found the limit for G2926B! It’s really hard to show the surface quality of a clear glaze in photos, but hopefully you can see that the last G2926B tile has visible glaze ripples.

I might do a third test series to find the limit for Clear Bright (I probably used less water in that glaze when I first mixed it in the big bucket). 

I added even more water to my big buckets and my pots are looking better and better. Here is a side-by-side comparison of details on older and newer NZ6 mugs:

 

Additional thoughts

Glaze recipes can be very specific and a difference of even 0.5% for some ingredients can have striking results. Why then do we wave our hands when asked about the amount of water or other consistency additives (e.g. epsom salts solution)? Unfortunately, a lot depends on the glaze application process, clay body, and bisque temperature. The more absorbent a pot is, the more water the glaze might need. I have a suspicion that I was so off with my water content because I bisque at cone 06 and people usually bisque at cone 04, maybe? Or just my clay body is weird? Or I use incorrect amount of epsom salts (the thixotropy still feels like black magic to me).  I’m not sure. Tony Hansen gives a lot of detail on how to properly prepare his clear glaze. I think I followed his instructions and still ended up with too thick of a glaze. If you are not satisfied with your clear glaze surface quality and you are using a popular glaze recipe that everyone seems to love, you might need to do an additional water “line blend” of your glaze (or figure out the magic thixotropy).