What causes crawling in pottery?

Written By Earth Nation Ceramics

Throughout my blogs I often stress the importance of measuring your specific gravity, especially with your handmade glazes; That being the ratio of minerals that make glaze vs the amount of water they are suspended in. While this is one of the easiest variables to measure within glaze tuning and designing, it is only one of the prime variables. In truth it’s specific gravity, application, clay absorption, viscosity, and temp are all just as important. Most glaze will work with proper application and specific gravity tuning but viscosity is the next big one we need to start keeping measurements of. Luckily for us the Ceramic materials Workshop teaches us how to do this with Epsom solution or Darvan giving us a way to measure this by way of a Zahn cup (I like to use Zahn cup #3). Keep in mind viscosity is a fluid's resistance to flow, essentially how "thick" or "thin" a fluid is. A fluid with low viscosity, like water, flows easily, while a fluid with high viscosity, like honey, flows slowly. A Zahn cup is a small cup with a little hole that will measure how long it takes for a fluid to get through the hole or break. The longer it takes to get through the hole the more viscose the fluid is. In our case we can use something like Epsom salt solution to bring up the viscosity of the glaze. Here is a short video with my newest glaze “Greymon v.2” showing massively different results by tuning the viscosity with Epsom salt solution and showing what happens when your application is too thick.

The main point of this video is to show that crawling is mostly caused by glaze being applied too thick. Well, as we just talked about, we can bring up the thickness/ viscosity of a glaze by using Epsom salt solution. Keep in mind Specific gravity is a different measurement from viscosity. Just because you have high S.G does not means you have high viscosity. While you can add water to a glaze to get a thinner application ( lower the viscosity, the measurements do not correlate in a 1:1 ratio.

Low S.G high Viscosity

This is a previous line test with varying specific gravity. The S.G is relatively low but I added about 10 ml of Epsom salt solution in each one. See the cracking? guess what happened to most of them….

  • fired to cone 6 ox

Results

All of them crawled.

It’s pretty obvious by the cracking we can see on application that this was going to happen.

  • fired to cone 6 ox

Greymon v 2

  • Application: 3 second dip

  • 1.4 S.G

  • b-mix slip cast tile

  • cone 6 30 min hold at 1800f

  • Darvan added

glaze good, color bad.

Greymon v 2

  • Application: 4 second dip

  • 1.4 S.G

  • b-mix slip cast tile

  • cone 6 30 min hold at 1800f

  • Darvan added

We can start to see some color in the glaze. I want that yellow.

Greymon v 2

  • Application: 4 second dip

  • 1.34 S.G

  • b-mix slip cast tile

  • cone 6 -30 min hold at 1800f

  • Darvan added

Greymon v 2

  • Application: 3 second dip

  • 1.34 S.G

  • b-mix slip cast tile

  • cone 6 - 30 min hold at 1800f

  • Darvan added

The difference between a 3 and 4 second dip is large to me. I see way more color in the above tile.

Greymon v 2

  • Application: 4 second dip

  • 1.4 S.G

  • b-mix slip cast tile

  • cone 6 - 30 min hold at 1800f

oh yeah. now were getting somewhere. It ran but that’s normal as I often overlap on the top.

Greymon v 2

  • Application: 3 second dip

  • 1.4 S.G

  • b-mix slip cast tile

  • cone 6 - 30 min hold at 1800f

Almost perfect.

Greymon v 2

  • Application: 3 second dip

  • 1.34 S.G

  • b-mix slip cast tile

  • Added 10 ml of Epsom salt solution.

That is the color I want. And at a lower S.G? Maybe next time I should add 3 ml or so of epsom salt solution while keeping the S.G at 1.35.

You will note that this was the tile that had a little bit of cracking on application. the crawling happened just as we assumed it would.

Greymon v 2

  • Application: 4 second dip

  • 1.34 S.G

  • b-mix slip cast tile

  • Added 10 of Epsom salt solution.

Dear god look at that crawling. And with only 1 more second of application than the above tile. soo much worst. Note that this was the side of the tile that had more cracking on application.

At the moment my conclusion is the majority of crawling issues are cause by thick application of glaze. We can make this happen by purposefully making the glaze thick even when keeping the same application.

Shout out to the Ceramic Materials Workshop for telling me this. Of course they are a great source of info but as always we must confirm it with testing ourselves.

Earth Nation Ceramics
Next

Tuning viscosity in glaze (thanks, I hate it)