Terra Sigilata

Today there was a workshop at the studio led by Mike Flaherty, whose show just opened at the Craft Council Gallery. He talked a bit about using lusters, and decals, but mostly it was about terra sigilata–basically very fine slip, which can used to give you a polished-looking clay surface[6].

Most interesting–for me, anyways–was his little side trek into underglaze territory. I use a lot of underglazes–it’s where I get all the bright colours in my Robot line from[5], and they’re come in particularly handy with this teacup project[1]–and I love them, but they’re sort of expensive. Getting around to testing out a few do-it-yourself underglaze formulas has been on my to-do list, but the commercial ones were always so… easy. And available. Also, don’t come with notes saying things like “will vary slightly between batches”[2]. And involve all these finicky steps for things that, when you get down to it, I would be mixing fairly often.

Mike started experimenting with underglazes himself for this project, but made his from the start. He found his initial results unsatisfactory and “odd”. Somewhere along the way, though, he chanced across a mention of terra sig being useful for underglazes. Just mix a dry white sig 50:50 with a stain or oxide[2], and voila! Underglaze. Reliable, wonderfully brushable on green and bisque, and really simple to make.

Now, he was working with lower temperatures than we do in the studio, so things may vary a bit at cone 6 and under my clear[4], but still. I have a few test pieces ready to be plopped into this week’s glaze firing. Maybe I’ll get something really, really useful out of this.

[1] Want an opaque shiny glaze of a particular shade I will probably never use again? I can either mix up a bunch of tests, fire them, tweak them, and mix up a bucket of whatever comes out closest to what I had imagined, or I can reach for a bottle or two of underglaze.

[2] Not copper carbonate. It does weird things, apparently.[3] Or at least, it does at his firing temp and with the particular clear glaze he was using over it; obviously everyone else’s mileage will vary.

[3] Oh, copper carb. You are so useful, yet so unpredictable…

[4] Crimson Mason stain, I’m looking at you…[5]

[5] The first ever Robot mug was made with a slip coloured with Crimson Mason stain. After firing, the inside came out white. That was the day I decided to switch to underglaze…

[6] To make terra sigilata: Find some dry clay. Slake it thoroughly, mix it well with a deflocculant (Darvan or sodium silicate work fine). Let sit overnight. Solution will separate into layers; bigger, heavier particles of clay will fall to the bottom. Siphon off top layer of water, top layer of fine clay particles. DO NOT SIPHON OFF THE SLUDGE! Then there was some stuff about specific gravity, but that’s irrelevant if you’re just drying the sig out to make underglaze.


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