I tried to make a glaze with an effect that looked like rust spots, as I am interested in weathering. In November I tried this on a test tile, with some success, but had difficulty scaling it up to a larger piece. As the recipe contained zinc and was prone to running, it struck me that this was like a matt micro crystalline glaze. I know that water content can be critical with crystals, so I decided to test the specific gravity of the glaze I had mixed.
First tests
I had added a lot of water to produce a thinner glaze, so removed most of it with a small cup. I mixed what was left which appeared to be quite thick. I used a large test tube to check the specific gravity (SG), and found that it was still only 1.31. As I had stirred the glaze I could only add water back to get 1.26 and then 1.21and put in test pieces for firing, I left an unglazed area at the base, in case of running.
The specific gravity testing was difficult as adding exactly 100ml into a very tall thin test tube was impossible, and it took many corrections to get the desired quantity. I think the syringe method would be easier. We don't have a syringe, Cath found one on the internet, so I think I may need to invest in one. It would be useful for removing the water too, if too much has been added. If I could simplify this process, it would allow specific gravity testing when going from test tiles, to full size pots.
Results - all three tests failed to get the desired effect, and the specific gravity is now at an average level of up to 1.45. I will try to get rid of more water and increase the specific gravity further.
Second tests
I have removed more water and completed two more tests at up to 1.7,
Results - the glaze is thick and runny. It is quite an attractive metallic brown colour, but no sign of the rust spots that I had hoped for.
Conclusion
I have now tested this glaze 7 times, with 6 different water content, and two different application methods. I have persevered for two reasons.
Firstly because I wanted the rust spots on the test tile. However, I think that I need to look at the recipe again firing cycle and compare it to other matt microcrystalline recipes. I need to check it carefully, especially any ingredients that can vary by country. Maybe I need to look at UMF/Seger formulas?
Firstly, and more importantly, because the specific gravity tests were really useful and I obtained a much better insight into the glaze making process.
When I usually add water to a glaze recipe I measure it carefully, aiming to produce a specific gravity of about 1.4, as this works for most glazes. However after sieving the glaze, I test it using my fingers and my eyes and if it looks a bit thick, I adjust the water to get a 'cream' consistency. This is a very inexact science, and specific gravity is relatively easy, and preferable to ruining one of my pieces. I have learned that you can't tell SG by finger dips or by looking at how it pours. I am going to measure SG on my successful glaze tests, so that as I scale up to a larger batch, I have a better chance of achieving the same results.
Secondly because I wanted the rust spots on the test tile. However, I think that I need to look at the recipe again and compare it to other matt microcrystalline recipes. I need to check it carefully, especially any ingredients that can vary by country. Maybe I need to look at UMF/Seger formulas, or is that just a step too far?
Comments