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Showing results for tags 'porosity'.
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I'm interested in experimenting with alternative porous materials suitable for slip casting, with useful properties such as flexibility, faster drying, increased durability, machinable, etc. Can anyone suggest plastics (foamed PVC, polyurethane, etc.) or other plasters (eg: Hydrostone) worth looking into? Thanks!
I was recently given a piece of a fired ceramic vessel by a friend who told me it was terracotta and asked me if I might be able to reproduce it. The main reason this vessel was made of terracotta was supposed to be its relatively high porosity, which would allow water to evaporate through the walls of the vessel. The piece of clay I was given felt a lot heavier than my garden flower pots so I decided to do a bit of research and run a few tests, in order to assess its "terracotta-ness". After some digging it seems that terracotta is hard to pin down. There appears to be a lot of qualitative discussion but little in the way of technical benchmarks that one could measure against. Any references or standards anyone knows of? I am particularly interested in where terracotta ends and stoneware begins. Many of the "terracotta" clay spec sheets I could find from vendors quote firing ranges (cone 5 and 6) which look much more like midfire stoneware than earthenware to me. Since I have always understood terracotta to be a kind of earthenware, I assumed it would melt or distort when fired to stoneware temperatures, but this did not happen. Before firing a piece, I accidentally dropped it and it splintered/cleaved into two pieces which keyed together perfectly over an area of about a square inch. I fired both pieces sitting next to each other in a strong reduction firing (mains gas) to cone 10 well down, cone 11 half down. Although the clay turned a noticeably darker chocolate brown color, it did not seem to distort or bloat much at all. In fact those two pieces still keyed together afterwards in virtually the same way they did before the firing! Essentially no change. I could only find one general reference to the porosity of terracotta (Hansen), with a suggestion that is should be approximately 10%. Does this sound right? The spec sheets I was talking about above quote porosities around 5.5% to 8.5% level, but again that is firing to midfire temps. The porosity of my shard seems to be between 5% and 6% when I tested it, which seems low to me, headed more in the direction of stoneware porosity. However 5% or 6% seems to be just right for mid-fired "terracotta." When I fired my piece to stoneware and again measured porosity, it had dropped to around 1%, which seems very close to the lower bound for a good tight stoneware body. So is terracotta simply a stoneware body fired to a midfire range? Or, or??? Thoughts?
Hello, I'm making some unique gifts and would like to know how I can change the porosity of terracotta slip. I would like a green colored one to be very porous, a blue one medium porous, an orange one less porous, you get the idea. I was going to slip cast into plaster molds. Any and all input is welcome at this point as I can't express the yawning chasm between my excitement to achieve this and the ability to do so. I have a very good idea on how to measure the flow rate once I have more control of the porosity. Thanks in advance for any assistance.