Technical moodle drop in session with our lovely technician Andrew Allum(Andy) about vitrified ceramics.
Q: Is vitrified ceramics food safe?
A: Yes. Vitrification is essentially densifying pores in clay to a non absorbent stage through firings to the fluxing temperature(flux: fusing agent in clay, usually feldspar), which would create a matte/satin finish and be food safe. However, I would advise going through few tests to confirm:
-test 1: marker stain
draw something with a marker pen on testing surface, the piece is vitrified if you can
completely wipe off the marker stain.
-test 2: boiling water
weight testing piece and record, then soak the piece in boiling water for a few minutes and
weight again, the piece is vitrified if it didn't get drastically heavier/absorbed too much water.
Q: how can you tell the difference between over-fired and vitrified ceramics?
A: Over-fired ceramics usually are bloated or have blisters because of the escaping gas, vitrified ceramics would have a matte/stain finish and non absorbent surface.
Q:What's the recommended firing temperature/time for vitrification?
A: it depends on the type of clay and the ingredients in it. If there's oxides or stain in clay, the gases it gives off can bring the temperature up 30-40°c. For instance, If you have manganese in a clay that fluxing temperature is 1170°c, it's the best to bring the firing temperature down to 1130-1150°c. You can also do a slow firing to avoid blisters from releasing gas by going through 80-600°c slowly (eg.100°c per hour) and soak for 15-30 minute at 1150°c
Q:What's the best clay for vitrification?
A: Porcelain Maybe? for the translucency and whiteness in common domestic-ware
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