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How to Calibrate Your Glazes to Prepare for Pottery Glazing

By Nan Rothwell, May 4, 2012 Read Comments (16)

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Calibrating glazes each time you glaze is a step that is probably skipped by a great many of us when we glaze our pottery. But it is a step that can help make glazing results more reliable.

In today’s post, an excerpt from her new DVD Glazing and Decorating Pottery, which ships next week, Nan Rothwell explains her process for keeping her glazes consistent from one glazing session to the next. – Jennifer Harnetty, editor.

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This clip was excerpted from Glazing and Decorating Pottery with Nan Rothwell, which ships next week from the Ceramic Arts Daily Bookstore!

For more great glazing techniques techniques, be sure to download your free copy of Three Great Ceramic Glazing Techniques: How to Formulate Successful Crystalline Glazes, Add Depth Through Carving and Layering, and Glaze in the Majolica (Maiolica) Style.

16 Comments on "How to Calibrate Your Glazes to Prepare for Pottery Glazing"

alessia t January 6, 2013 at 9:24 am -

Hi Nan, thanks for this great video. Could you explain a bit the test with the finger. I am pretty new and I’m interested in any tips to have better glaze.
Val Rossman May 12, 2012 at 8:09 pm -

Nan, Thanks for taking time to reply. Your video clip and your answer are really excellent information. I’d been thinking about moving to a hydrometer, I’d been relying on either the finger test or a scrap piece of bisque to test. Thanks again!
Nan Rothwell May 12, 2012 at 7:33 pm -

Hi Val – Assuming that your glaze has the weight (hence the amount of water) you have used before, but its viscosity is off, you can treat it to thicken or thin it as needed. To thicken or flocculate a bucket of glaze, you can add Epsom salt. I keep some dissolved in water. Just a small amount will alter a bucket of glaze. It lasts a while, but is not a permanent fix. If your glaze has the right weight, but seems too thick, you can thin or deflocculate it by adding sodium silicate or Darvan.
Val Rossman May 11, 2012 at 11:49 pm -

Nan, I couldn’t hear on the video what you said about altering viscosity. You said if it was thin add epsom salt but I couldn’t hear what you said to add if it was too thick. I listened a couple of times but couldn’t catch what you said. Thanks for sharing your expertise. Val
Nan Rothwell May 8, 2012 at 7:31 pm -

Nick — that is a fabulous idea. I am going to switch to using a syringe the next time we need to calibrate glazes. I love sharing and learning new ideas like this. Isn’t the internet grand? Thank you! As for the question from bechtk123 — I use the same scale for recording the weight as for weighing out batches of glaze. It is an electronic gram scale that is quite accurate, even at low weights. I used a balance-beam scale for many years — also accurate but more time consuming.
Nick Molatore May 7, 2012 at 2:28 pm -

Very good article. Regarding use of a hydrometer. This does not work reliably because of the thixotropic characteristic of glaze. Many glazes thicken within a few second s of being stirred. This complete throws off a hydrometer. I have found a MUCH quicker way to accurately weigh a glaze. Get a large syringe from a farm store. I use one that holds 50ml that cost only 2 or 3 dollars. You can accurately fill it with exactly 50 ml of glaze and then weigh the entire syringe. If you want, you can weigh the empty syringe and then subtract this to get the weight of the glaze, but this really isn’t needed because what you are interested in is being able to reproduce the consistency of the glaze over time. The syringe can be quickly and accurately filled and it is easily washed. I get all my glazes the consistency that I want and record the weight of 50 ml of each glaze. Now when I am ready to use the glazes that have sat for weeks, I can quickly check to see if water needs to be added. Better yet, in a shared studio or classroom situation, the glazes can be easily maintained at the desired consistency.
bechtk123@gmail.com May 7, 2012 at 11:35 am -

This is very clear and very useful info. My question is how sensitive is your scale? Thanks so much.
Nan Rothwell May 5, 2012 at 6:52 am -

Thanks for the comments and questions. I’ll try to respond to them in the order they arrived. Please keep in mind that my responses come from practical studio experience, not from a deep academic understanding of glaze chemistry. Lisa – if the weight has not changed, but the glaze is behaving differently, then something has happened to the electrostatic attraction between the particles of material suspended in the bucket. As I understand it, this is caused by organic and/or soluble components of the glaze acting over time to change the viscosity. In my studio, the glazes most prone to needing readjustment contain wood ash, soda ash, or nepheline synite. Janine – You are right – removing water from a glaze that has settled can indeed cause a change, especially if your glaze contains many soluble materials. As a practical hedge against the problem, I tend to mix new batches of glaze rather thick and then gradually add water. If I guess wrong and have to remove water from a glaze with soluble materials, I try to keep the removed water (supernate) in a plastic container near the bucket, so that I can use it to rehydrate the glaze as needed. Over time, you need to rehydrate and then recalibrate… Shawn – I don’t know what size cup it is – sorry! I bought it in the paint supply section of my local hardware store and they had only the one size. But I suspect that absolute numbers don’t matter. It’s like the size of your container for measuring the weight, the important thing is to be consistent. Nigel – I am glad that a hydrometer is working for you. I’ve never had much luck with hydrometers. The same bucket of glaze and same instrument would give me different consecutive readings. I don’t know what I was doing wrong, but the inconsistent readings made me distrust the process, and I gave up on it some years ago. Perhaps you or other readers with positive experiences with hydrometers can set me straight?
nigel carpenter May 5, 2012 at 1:10 am -

Seems very complicated and time consuming, its far simpler to use a Hydrometer and record the measurements. This will measure the specific gravity of the liquid – quicker and less fussy. Too heavy, add more water, too light let the glaze settle and syphone off some liquid.
Clifford Ross May 4, 2012 at 5:29 pm -

Am I the only person who can hear NO sound at all???
Liliana Cejas May 4, 2012 at 4:32 pm -

Los videos son muy buenos, pero yo sólo puedo verlos pero no entiendo lo que dice pues NO SE INGLES, me parece que sería muy interesante que los VIDEOS tuvieran sus sub titulos o traducción en español, para así poder realmente enteder lo que están diciendo. Ya que son sumamente interesantes para aprender. Les dejo la sugerencia, quizas muchas personas como yo NO SABEN INGLES y necesitan los subtitulos abajo en español, además que creo que puede haber muchas personas que están igual que yo no sabiendo ingles- Muchas gracias.
Shawn George May 4, 2012 at 3:12 pm -

Just the info I was needing! What I thought I had to do was way left field. This I can do! I noticed there are diff sizes of zahn cups. What size are you using Nan? Thank you for this!
Shawn George May 4, 2012 at 3:09 pm -

I have been needing just this info! What size zhan cup are you using Nan? I noticed there are many diff sizes.Thank you very much – what I thought I needed to do was way left field! This I can do!
Janine Wood-Bokman May 4, 2012 at 12:44 pm -

Thanks for the great video. If you determine your glaze has too much water in it and you pour some off after settling, is there a possibility that some material might be in solution resulting in a different formula?
lisa steinberg May 4, 2012 at 12:41 pm -

This was great, and I am going to start doing this TODAY! One question though: if the viscosity measure is significantly different, (but not the weight) what caused that?
quammen@mindspring.com May 4, 2012 at 12:39 pm -

As a self-taught potter, I’ve learned these things through many, many years of research and trial and error (and error and error). Wish I’d had this info 30 years ago! Thanks, Nan.