Terra Sigillata - strike 1
Another simple way of affecting the surface of a clay body is with terra sigillata (latin for "sealed earth"). It's basically a very thin liquid slip, i.e. tiny particles of clay suspended in water through the use of a deflocculant. The deflocculant is an electrolyte that ends up changing the electrical charge on some of the clay particles so that they are not attracted to each other and therefore don't gather together in clumps or "flocks", and instead they float individually in the water. What you end up with is a liquid that you can paint on your greenware and then polish to get a shiny surface on your fired piece. For a lovely example, see this shiny Greek vase:
The fun part was getting the clay powder, since I have none of that either. So I made up some noodles, let them dry, then pulverized them and passed the powder through a 200-mesh sieve which is unbelievably fine.
I more or less followed this recipe, but adapted it slightly to make more. In a 2 L bottle, I put 1.9 litres of water and 2 g of trisodium phosphate and mixed it up. I then added 400 g of clay powder, a little at a time, and let it slake down to the bottom.
After about a week I noticed that I wasn't really seeing the 3-part separation that most recipes indicate you should see - sediments at the bottom, murkier water in the middle, clear water up top. And once I took out the middle part of the water and tried applying it to my little unfired thumb pot, I realized my first attempt at making terra sig was a failure.
I think I probably left it to sit too long and so all the particles ended up settling, or maybe the bottle was too small and I couldn't mix up all the ingredients sufficiently. In any case, I'll try again.
Most terra sig recipes I found called for clay powder, water and a deflocculant like sodium silicate or "old-formula Calgon" (the kind with lots of phosphates) but since I had neither, I found some heavy-duty tile cleaner at the hardware store which is trisodium phosphate, and used that. Turns out others have used it too, so I was happy to have my phosphate-intuition confirmed.
The fun part was getting the clay powder, since I have none of that either. So I made up some noodles, let them dry, then pulverized them and passed the powder through a 200-mesh sieve which is unbelievably fine.
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I more or less followed this recipe, but adapted it slightly to make more. In a 2 L bottle, I put 1.9 litres of water and 2 g of trisodium phosphate and mixed it up. I then added 400 g of clay powder, a little at a time, and let it slake down to the bottom.
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| floating clay powder |
After about a week I noticed that I wasn't really seeing the 3-part separation that most recipes indicate you should see - sediments at the bottom, murkier water in the middle, clear water up top. And once I took out the middle part of the water and tried applying it to my little unfired thumb pot, I realized my first attempt at making terra sig was a failure.
I think I probably left it to sit too long and so all the particles ended up settling, or maybe the bottle was too small and I couldn't mix up all the ingredients sufficiently. In any case, I'll try again.
Do you know know where you went wrong? very interested!
ReplyDeleteHi Mamo, I'm pretty sure it was simply that I left the mixture sit too long. The second time I did it I was more careful in watching the process daily. I'll double-check my pen-and-paper notes to see if there was anything else I did differently, but I think that was it. Good luck and let me know how it goes.
ReplyDeleteOkay, I found and checked my old notes, and I did this:
ReplyDeleteadded TSP to warm water, shook vigorously to mix.
added all clay to mix, shook vigorously to mix further.
mix:
100g clay
300g water
.5g TSP
So it was all small quantities, but I think the order of adding the material and shaking it all vigorously was important. Then not letting it settle as long as the first time, too. I'll get back to this stuff eventually, because it certainly bears repeating to learn it better, fer sher. Good luck.