Glazes are mixtures of clays, minerals, and water. Their Specific Gravity (SpGr) tells you how much dry material is in the glaze; commonly thought of as how “thick” the glaze is. A value of 1.4 indicates a pretty thin glaze, whereas 1.7 is pretty thick. Terra Sigilatta is very thin, it has an SpGr of about 1.15; while casting slips are thick, measuring out around 1.75.
Checking the SpGr helps avoid over or under thickness while glazing. Over thickness, of course, is one of the big reasons for glaze runs. Under thickness often leads to dark, muddy results. Having the right SpGr is important to getting consistent glaze results.
A good way to measure SpGr
Figure 1. Set-up and TARE
1) Place a dry, empty 100ml graduated cylinder (located in the Glaze Room upper cabinet) on the studio scale with enough added weight so that the total adds up to about 0.5 pounds. See Figure 1; click to enlarge. The added weight is needed because the minimum weight for scale accuracy is 0.44 lb.
2) Push the TARE button located on the right side of the keypad. The weight displayed will then drop to zero.
3) Carefully pour 100ml of glaze into the cylinder, and place it back on the scale. Wipe off any excess water or glaze; these create sources of error.
Figure 2. Multiplier converts lbs to SpGr.
4) Use the keypad to enter the multiplier 4.54. This converts the glaze weight into grams and divides by 100 in one step. The Specific Gravity (SpGr) then automatically displays to the right as shown in Figure 2. In this case, it’s 1.41.
5) Compare the result in step 4 to the target SpGr written in the Student Glaze Book (Located in the 2nd drawer of the Glaze Room filing cabinet.) Each glaze is listed on a separate page, and cataloged in alphabetical order.
If your glaze is more than 4 or 5 points too thick or thin, discuss how to proceed with your teacher or a studio potter ! Don’t try to adjust the thickness of the glaze yourself.
Figure 3. Student Glaze Book
In this case, the glaze is Jasper Tan with a target SpGr in the glaze book of 1.42, noted just below the glaze name, so the thickness is near perfect…..just 1 point on the thin side.
At CAW we bisque to cone 06 (1823F), and adjust our glazes to a SpGr usually between 1.4 and 1.6 .
Dry Mix-to-Water Ratio:
It’s sometimes handy to know the ratio of dry ingredients to water in a glaze. For example, a SpGr of 1.5 does not actually mean there’s 50 g of dry mineral in each 100 g of water. That mixture would weigh 150 g, but the volume would be larger than 100ml. That’s because the water alone takes up 100ml of space; the dry mix adds more volume.
A good way to find out the amount of dry mix in a glaze.
Weigh one cup of glaze, and let the water evaporate.
1. Place an empty, dry container large enough to hold one measuring cup of glaze onto the studio scale, weigh it, and push the TARE button.
2. Pour one measuring cup of glaze into the container. The weigh shown will be the weight of 230 milliliters of glaze. Write down the weight.
3. Let the cup fully dry out (takes a few days), then weigh the cup again. That weight is the weight of the dry ingredient alone, while the difference in the two weights is the water weight.
Ballpark numbers for CAW glazes are between 250 to 300 grams of dry mix per measuring cup (230 ml) of glaze. Since there’s four cups in a quart and four quarts to a gallon, you need 4000 to 4500 grams of dry ingredient per gallon of desired glaze. That means there’s 35 to 40 lbs of dry mix in each full bucket of glaze. For the glaze in Figure 3, Jasper Tan , ingredients are calculated in the book for making 1, 2, 3, & 4 gallons of the glaze.
Interesting Note: The weight of the dry minerals exceeds the weight of the water in a given volume of glaze !