|Monthly Tech-Tip |
The commercial cone 04 clear brushing glaze on the left works really well on our bodies so I sent it away to be analyzed (about $130). From that information and using my account at insight-live.com I was able to create a recipe, having the same chemistry, employing two Fusion frits (which amazingly supplied all of the fluxing oxides). In this cone 04 melt fluidity comparison they are almost identical (mine, G3879, has a little more surface tension). The Al2O3 and SiO2 levels would make, even a cone 6 glaze, jealous! So it should be very durable. And it has low thermal expansion (no crazing). With the bucket of dipping-slurry I made I can glaze a piece perfectly evenly in seconds rather than the normal 10 minutes of fiddling with a brush and three coats! I have used it on dozens of pieces, it's amazing. One caution: It is possible to duplicate a transparent glaze like this but not a coloured one (a lab could not analyze the complexities of the color, stain colors are about more than chemistry (firing method, particle physics). For colored glaze you have to do trial-and-error testing with stain additions to this base.
I have 200 mesh powdered samples of ceramic glazes, mixes of minerals and man-made glasses. I need to know the percentages of B2O3, ZnO, Li2O, BaO, Bi2O3, MgO, CaO, Na2O, K2O, PbO, P2O5, SrO, ZrO2, Al2O3, SiO2 (some of these might not be present).
Here are some of the papers I had to deal with to have a mystery colourless transparent glaze analyzed (so I can calculate a mix of my own materials to produce this chemistry). Since no one test will find all the 13 common oxides that may be present in low temperature colorless glazes I had to order three different ones (I also asked for bismuth). It ended up costing about $130. I found this lab was the best one to deal with (because ceramic materials are outside their experience, others worried about ruining their platinum crucibles). A couple of things to remember: Emphasize that you want the results in oxide format (not elemental). To avoid sample preparation costs, dry out some of the glaze slurry and crush in into a homogenous powder sample using a mortar and pestle (they only need a few grams, I did 20).
Notice the water has wicked up to about 1 cm from the rim (the piece sat in water overnight). The glaze fits so there are no cracks for the water to seep through. However, being fired at cone 04, the body is quite porous. The piece has a unglazed base. Notice the water even travelled up the handle. Less exposed bare clay on the base would improve the situation somewhat, however it would be much better to choose a body that vitrifies sufficiently dense so that it does not absorb water (or fire to a higher temperature). There is a not-so-obvious issue here also: Although this piece did not explode in the microwave, it got incredibly hot. Amazingly, through all of this, the glaze has not crazed. It is G3879.
G3879 - Cone 04 Transparent Low-Expansion Base Glaze
A super transparent low fire base clear glaze created by reverse engineering a commercial product.
Five low fire glazes: Which is the best?
Frits are used in ceramic glazes for a wide range of reasons. They are man-made materials of controlled chemistry with many advantages or raw materials.
Kovar metal is thermal expansion compatible with borate glazes. You can actually glaze and fire it like pottery at cone 05, 06!
A Low Cost Tester of Glaze Melt Fluidity
This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.