[Clayart] Measuring Glaze Thickness

Mon Feb 17 14:24:52 UTC 2025

Hi all,
Lot’s of talk about the Specific gravity and viscosity of Glazes, etc., so I thought I’d share my methodology.

  1.   Go to your local pharmacy or better yet, hospital pharmacy and buy a 60-100 cc syringe. Get the one with a large nozzle, for use with a catheter.
  2.  Go home and Mix up your glaze. Mix it light on water, thicker than you want the glaze in its working state.
  3.  Weight your newly purchased syringe while empty (in grams) and write down the weight. TAR your scale.
  4.  Suck up 50cc of glaze into the syringe and weigh it (in grams). Subtract the empty weight from the 50cc weight and multiply by two. You now have a specific gravity measurement. Wash out your syringe and dry it for next time.
  5.  I mix my glazes short on water and Let sit overnight before sieving and testing specific gravity. Then I add water until I get my target SPG. Then add Darvan 7 or Sodium Silicate or Epsom salts as needed to get the viscosity and workability I want for a specific glaze.
  6.  Repeat this SPG measurement every time before you use the glaze. Adjust glaze to your target.
  7.  Each glaze is different, each glaze has it’s own working state. Each potter is different, each potter has their own working state.
  8.  Pottery process is challenging and wonderful!

Paul

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________________________________
From: Clayart <clayart-bounces at lists.clayartforum.com> on behalf of Robert Santerre via Clayart <clayart at lists.clayartforum.com>
Sent: Monday, February 17, 2025 12:07:04 AM
To: 'Clayart international pottery discussion forum' <clayart at lists.clayartforum.com>
Cc: Robert Santerre <rfsanterre at gwi.net>
Subject: Re: [Clayart] Measuring Glaze Thickness

Yeah Dick, this same method of measuring specific gravity of a glaze can be done quite precisely with a 100 ml graduated cylinder (glass or plastic) that can be readily purchased online.  And if the cylinder gets broken, it can be readily and cheaply replaced.  Weigh the empty cylinder, then fill it with water to the 100 ml mark and weigh the filled cylinder.  Then pour out the water, fill the cylinder to the 100 ml mark with your glaze mix, weigh it and make the calculation ... weight of the glaze mix minus the weight of the empty cylinder divided by the weight of the water minus the weight of the empty cylinder = specific gravity of the glaze mix.  Quick and easy, very repeatable and quite precise/accurate.

But, to be clear, specific gravity does not describe the "thickness" of a glaze mix.  Thickness or viscosity describes the rheological property of a glaze. It describes how the glaze flows, a mechanical property of the glaze.  The “thickness”, flow or mechanical property of a glaze can be influenced in many ways (independent of its specific gravity).  For example, a glaze with a good amount of clay in it can be thickened by adding small amounts of acid, like hydrochloric acid … or the thickness (viscosity) can be decreased by adding small amounts of a base, like sodium hydroxide. You can get a reasonably accurate measure of glaze “thickness” by pouring a measured amount of a glaze into a funnel and timing how long it takes for the glaze to pour out of the funnel.  The longer it takes to empty out of the funnel the thicker, more viscose the glaze.

The two measurements (specific gravity and “thickness”) work together to determine how much glaze you will apply to a pot.  In my experience glaze viscosity is often the most important factor determining how much glaze you are applying to a pot.  There are of course a number of other factors influencing the amount of glaze applied to a pot, e.g., how long the pot is held in the glaze, is the pot dipped in the glaze or is the glaze poured over the pot, variations in the thickness of the bisqued pot (for example the thickness the handle of a mug versus the wall thickness, etc., etc. Lots of variables, many of which can only be determined and controlled by the experience of the individual potter.

Again, in my experience that’s why glaze formulas don’t always travel well.  The experience of the potter and his/her kiln are THE MAJOR FACTORS.

Bob Santerre

formerly Arrowsic Island Pottery

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-----Original Message-----
From: Clayart [mailto:clayart-bounces at lists.clayartforum.com] On Behalf Of Dick Lumaghi via Clayart
Sent: Sunday, February 16, 2025 5:58 PM
To: Clayart international pottery discussion forum
Cc: Dick Lumaghi
Subject: [Clayart] Measuring Glaze Thickness

The recent mentions on Clayart of the Shaner’s Red glaze prompt me to respond.  That glaze, along with one or

two varieties of a Cushing Mat glaze, gave me fits when I tried to measure its thickness accurately.  I tried various

hydrometer methods—the cheap and dirty use of a stick with a bolt screwed into one end with gradations along

the stick, or a battery tester with the bulb—and they just didn’t work for me. I happened to bump into a potter who

did the weighing method and it was transformative for me.  Yes, it took a bit of time to check all the buckets of

glazes at the beginning of “Glaze Day”, but the results were, for me, wonderful: FINALLY some certainty about

these most finicky glazes, and now I use it with all my glazes. Here’s my method:

I got lucky and found a generously sized, double walled plastic coffee cup with a good handle.  It holds about 500

grams of glaze and that seems about right to get consistent results.  Most importantly, the cup happened to have a

rather sharp top lip, where the two walls were joined together. So what I do is place my triple beam scale on a box

resting on the sink table so that when the cup is on the scale, its top is eye level.  I stir up the glaze with a Jiffy mixer

and a drill, pour a good amount into the cup, place the cup on the scale, and finish adding glaze with a turkey

baster until there is a nice meniscus (the bulge from the liquid’s surface tension trying to hold the liquid inside

the container) and then I read the weight and note it down. That sharp edge of my cup seems crucial to holding

exactly the same amount time after time.  When I get a weight that works, I put it on the glaze notes I have on the

wall and on the bucket as well.

Perhaps this is no news to most of you and I cannot claim any originality with this method.  I did, however, have

a (rare) flash of inspiration that may be novel.  It occurred to me that if I somehow lost or broke my precious cup, I’d be

sunk—all that work having to be done again.  I was a pretty rotten math student, but something must have remained,

for I got the idea to weigh the cup full of water and note that weight in my records so that I could have a factor to measure

against any other container in the future.

I’ve been doing this method for over 40 years and it occurs to me that perhaps there are more up to date ways to do this

and I’d be open to hearing about them.

Thanks.

Dick Lumaghi

dlumaghi at cybermesa.com=

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