Specific Gravity for Glaze

This page describes how to adjust the specific gravity of ceramic glazes. It is intended for folks with some glaze mixing experience.

Why would I need to change the specific gravity of my glaze bucket?

You may not need to. If your glaze has a higher specific gravity than the manufacturer recommends, but it still looks great fired, leave it alone. If the glaze is just way too thick and you know adding a flocculent will not help, it may be time to add some water and reduce the specific gravity of your glaze.

How did the specific gravity get so high?

It could have been initially mixed with a high specific gravity. Another possibility is evaporation. If a glaze bucket is left uncovered, water will evaporate. If you want to bring that glaze bucket back to the specific gravity recommended by the manufacturer, you can use the form below to calculate how much water to add.

How do I measure the starting specific gravity of my glaze?

buy a 100ml graduated cylinder. get one made of glass. it's worth the extra money.
buy a gram scale that can weigh a kg.
turn scale on
put your graduated cylinder on the scale
click tare to zero out the scale and ignore the weight of the cylinder
pour 100ml of glaze into the graduated cylinder
specific gravity is the weight in grams divided by 100. normally the weight of the glaze in the cylinder should be somewhere around 145.

Where do I get the target specific gravity for my glaze?

This should come from the manufacturer of the glaze. It may come from a glaze recipe. Whoever mixed the glaze should be able to help you determine the target specific gravity.

How do I get the total bucket weight?

weigh it on a scale. my studio only has a scale that weighs in lbs.
convert it to grams

How do I get the empty bucket weight?

put an empty bucket on a scale and weigh it. it should be identical to the bucket containing the glaze.
convert it to grams

OK, I put the data in the form and hit Submit... Now what do I do?

There are two things you can do. If you're nervous and you want to verify the specific gravity adjustment on a small scale without screwing up a whole glaze bucket, then do this...

pick the row in the table that is nearest your target specific gravity (probably one of the last two rows)
the first column of the output is water added to a 100ml graduated cylinder. add this much water to your graduated cylinder. you might need a syringe if you want to be crazy accurate.
mix thoroughly
pour the contents into a cup and then fill the graduated cylinder to exactly 100ml, weigh it and verify the specific gravity is what you expect

You either built confidence in the step above, or you blindly trust stuff on the web, either way you're ready to adjust the specific gravity of the whole bucket. The steps are very similar to doing so with a graduated cylinder, except at larger scale...

pick the row in the table that is nearest your target specific gravity (probably one of the last two rows)
the third column of the output is bucket water to add. add this much water to your bucket
mix thoroughly
verify the specific gravity with the graduated cylinder if you like, or move on to the next bucket.

Will this work for removing water from a glaze bucket?

Yes, but you probably shouldn't. Many chemicals can remain in the water of a glaze bucket and will never settle out. This means you're changing the chemistry of your glaze and not just removing water. If you really want to do this... let the solids settle to the bottom of the bucket. Remove the correct amount of water as directed in the table. Just make the starting specific gravity a smaller number than the target specific gravity.

Will this work with slip instead of glaze?

Yes.