Old Glazes Cracking and Crawling

[KachemakKaren]

I tried to search the forum for this topic because I'm sure it's been discussed but didn't turn up anything useful. I would greatly appreciate some help.

Being a hobby potter, I don't use up my glazes very quickly and have some old (5+ years) commercial dipping glazes that are now cracking as they dry on the pot, and/or crawling in the firing. Is there some way to alter the chemistry to make them useful again? 

Karen

[Callie Beller Diesel]

It’s probably just that some of the water evaporated out, and the glazes are thicker. A thicker application can easily cause the problems you’re describing. It’s fixed easily by adding some water back into the bucket. 

The question will be how much water, because if you thin it down too much, it might not look how it’s supposed to. While you can eyeball it if you know your materials enough to know what they’re supposed to feel like, if you want to bring it back to original, a little math and measuring to find the specific gravity is in order. Find your existing SG, and add water until it’s back down to the manufacturer’s original recommendations. If you bought this glaze as a dry mix, the SG may be in the mixing instructions. If you bought it fully premade, you can call the supplier or check online for this information. 

To measure SG you need a gram scale that preferably measures accurately to 1/10 or 0.1 of a gram, and either a clear plastic drinking cup or a large veterinary syringe that can at least 50 ml. A 100 ml syringe is ideal, but usually you need to hit up a large animal vet or Amazon for those. 

Mix your glaze thoroughly, so that all of the solids are out of the corners of the bucket and there are no lumps. Put your chosen measuring device on the scale and tare it out. 

If you are using a veterinary syringe, draw 100 ml of your glaze up, and wipe it clean and dry before you weigh it. If you use the 50 ml one, multiply the weight in grams x2 to get the weight of 100 ml. Divide the weight of 100 ml of your glaze by 100, and you will have the specific gravity number. So if your 100ml syringe weighs 170g, your specific gravity will be 1.70. If your 50 ml syringe weighs 80g, 80x2=160g. 160/100=1.60. 

If you don’t have or can’t get a vet syringe, take a clear plastic cup that’s as narrow as you can get it (better accuracy), and weigh out 100g of water. Mark the bottom of the meniscus, and you have a 100 ml measuring cup that’s calibrated to your scale. Dry it out, and weigh as above. 

There are math equations out there to figure out exactly how much water to add, but I tend to add water a bit at a time, mix thoroughly and re-measure. If you would like the equations, let me know. I think I have a calculator saved somewhere.

[Bill Kielb]

Just to add as above and……. Folks will often measure, record, then add a bit of water, measure again and test dip using their dipping practice to see if they like the application. Just a way to sneak up on what SPG is ideal for that specific glaze and your glaze habits.
Often one will come across a glaze that is working perfectly, that is usually a great time to record what that ideal number might be for future reference. While not universal, dipping glazes tend to be around 1.4 SPG. There are many, many exceptions to this though, but it’s not a terrible rule of thumb when all other things are unknown to find a starting point. Higher number = thicker glaze, lower number = thinner glaze (more water in it).

An example of why this can be personal and glaze dependent:  For me - I like my clear glazes to go on as thin and even as practical so they remain as clear as practical. So for me and my dipping habits, 1.38 is a good number with a three second dip for my clear glazes.

[KachemakKaren]

Thank you both so much. Last time glazing I mixed up some new glazes (I buy 5 lb bags of dry, usually Clay Arts Center or Coyote glazes) and did adjust them to specific gravity. I also altered the older glazes at that time, adding water to the recommended SPG, but two of them still crawled badly. The one I especially like is transparent, so I'll try adjusting it to a lower SPG and see if that helps. Will be working on that in the next few days and let you know how it goes. 

And many thanks for the detailed refresher on how to do the SPG! 

[KachemakKaren]

Just want to say adding more water to the problematic glazes eliminated the cracking and crawling. It was surprising how thin they needed to be. Thanks again!