Specific Gravity: Clay & Glaze

Note: This article is from Jesse Wiseman Hull’s website, now archived here.

The specific gravity of a liquid is simply the weight of the dry components in relation to the water. If the specific gravity of your glaze or slip changes too much, you could be applying too much or too little, which may prevent you from getting repeatable results. Finding the correct specific gravity for your methods is part of a trial and error process. As a starting point, the target for most glazes and slips is between 1.50 -1.80. Once you have found what works best, taking and recording a measurement will allow you to adjust the glaze as it loses water during use, storage, etc.
I usually try to keep my glazes within a “certain window”, and use the specific gravity as a guide for returning to, when things get far enough off. Having said that, some of my glazes are extremely application sensitive (Reddn’d Rainbow is a good example), so I’ll take particular care to be accurate with those.

Since the weight of 100ml of distilled water equals 100 grams, the best way to get a reading is by taking a 100ml sample of your glaze or slip and weigh it using an accurate scale and a graduated cylinder.
Tare the scale to the weight of the graduated cylinder, and carefully fill it to the 100ml mark.

(100ml water = 100grams)
100ml slip/glaze = 160grams.
The specific gravity is said to be 1.6, as it is 1.6 times the weight of water.

If the Specific Gravity is too high, you can lower it by adding water.
Take care to add only water (not gum solution). The binder or suspension agent does not evaporate with the water, so the water content alone is all that needs adjusting. Also, understand that it takes much less water than you think to thin a glaze, so adjust a little at a time.
If the specific gravity is too low, add more dry materials in the correct proportions regarding the clay or glaze formula.

Temperature also plays a role. In the winter, I have my studio thermostat programmed for a lower temperature during the late night/ early morning hours when I’m not working. I have therefore noticed a difference when measuring the specific gravity first thing in the morning, as compared to later after the heater’s been running for several hours. I get past this by blending the glaze for a few minutes each time I take a reading. Be careful not to blend too long, as the high shear friction can cause the liquid to heat up and skew the reading.
To be really precise then, the readings should actually be done with a thermometer as well.

In a future post, I will discuss how Viscosity/Rheology factors into all of this.

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