I agree about adding silica and alumina to fix crazing, if the base can support the amount needed to do so. Otherwise you need to change the fluxes, obviously not possible with a commercial glaze.
I think there is another way of looking at coe figures and their relationship to crazing. That being to look at glaze “families” or “systems”.
This is from digital fire:
Results are determined by the set of expansion numbers (different values are available from different sources) and the method of additive calculation method chosen (based on formula or mole%). Thermal expansion values predicted by calculation are relative (not absolute) and apply within 'systems'. Thus, if a glaze calculates to a higher expansion than another, and is in the same system, then it is more likely to craze. For example, if you have a dolomite, whiting, feldspar, kaolin, silica glaze and you try a bunch of variations, the calculated expansions will give you an indication of which variations have higher and lower expansions. But if you introduce lithium carbonate, or boron frit, or zinc, for example, now you have a different system. Also, some oxides, like Li2O or B2O3 do not impose their expansions in a linear fashion, thus they do not calculate as well.
I use a clear with a coe of 5.76 (using Insight). It is bombproof and does not craze on my clay, have used it for many years. Using the same clay I recently ran some tests for a friend with a glaze that has a coe of 7.20. I tested it on the same porcelain clay that I use the low expansion glaze on. To my amazement it does not craze either. I ran exhaustive crazing tests on it and all samples are craze free.
Neil E. got me thinking about this when he posted his clear glaze for porcelain a few months ago. At the time it didn't seem logical to me why it didn’t craze since the coe came in fairly high. Now it makes sense.
I think glaze blisters are one of the harder things to narrow down the cause of since there are so many possibilities for what the culprit is. If you feel like doing some reading there is a link to a Ceramics Industry 4 page article on blisters below, hopefully something seems a logical avenue to explore to fix the problem.
Paul Lewing posted that it's having glazes with differing silica:alumina ratios.
In Topic: How To Stop Glazes From Bleeding Into One Another?
25 March 2016 - 08:41 AM
The solutions to your problem that have been proposed have all been process or physical fixes, but the root of your problem is chemical. If you want glazes to bleed a long way into each other, they need to be very dissimilar. Conversely, glazes that are similar will bleed into each other less. And the key area of difference is Si:Al ratio. If the two have ratios have difference from each other that is greater than 6 they will definitely bleed a lot. For instance if one has a Si:/Al ratio of 5 and the other has a ratio of 12, expect a lot of bleeding. This of course makes sense. One is deficient in silica, the other is deficient in alumina, so the go looking for what they need in the other.
So... bottom line, if you want no bleeding at all, your best bet is color variations on the same glaze. Doesn't matter what glaze it is. The the only variable will be how much of a flux or refractory the colorant is in each different color variation.
I also think titania either from rutile or titanium dioxide helps.
A really good book for pricing and selling art with a Canadian perspective (tax implications etc) is Chris Tyrell's "Artist Survival Skills". It's geared towards 2D art but just about all his thoughts and philosophies are applicable to 3D also. He teaches professional development at Emily Carr in Vancouver BC. Super funny guy, if you ever get a chance to go to one of his speaking engagements it's well worth the time.
Pricing out my work now for this summers markets. Canadian dollar weakness has me marking my prices up by about 5% since so much of my materials comes from the US. Anyone else in Canada marking up prices because of our dollar?
Just because you fired the clay to over 2000F degrees it doesn’t mean it’s oven safe. How long did it take to go from room temp to 500F? A lot longer than the 10 minutes or so an oven takes.
If your clay is sold as porcelain and fired to maturity it’s going to be pretty tight, I’ve found tight bodies and large flat surfaces don’t do well in the oven. The outside edges heat up before the middle part of the pot and crack. Shape makes a big difference with ovenware, no tight corners, rounded bottom, even walled thickness, well fitting glaze covering all but the foot ring if possible. If you do think it’s okay to put your pots in the oven then covering the base with food, putting pot in cold oven then turn oven on so pot and oven heats up together, don’t add liquid to a hot pot.
I gave up making oven ware about 10 years ago. Whatever you say or print on care cards doesn’t matter, a lot of people think all pottery / ceramics is the same. If they can put their terracotta or stoneware pot in a very hot oven oven then why not a porcelain platter? I’ve had past customers think they can put pots under the broiler too, and one who put a frozen puff pastry covered brie in one of my pots in a 450 oven and complain that it cracked.
As for the microwave what is the definition of microwave safe? does it mean it won’t break or crack in the microwave? does it mean it won’t leach harmful materials when used in the microwave? does it mean the pot won’t get too hot to pick up when used in the microwave (like terracotta mugs handles do)? or does it mean the glaze won’t spark in the microwave? all of the above? If you have used your pots in the microwave and didn't have any of those problems then I would say they are okay.
Food safe? Probably okay but don’t know for 100% sure unless you get them lab tested. Cadmium is turning up in more and more underglazes (and glazes), there is going to be some chemical interaction between the underglaze and the glaze. Probably a negligible amount but again have to test to know for sure. I'm guessing your red, bright yellow and probably the purple and orange have cadmium.
Different opinions on crazed ware being sanitary. Some say if the clay is vitrified and good housekeeping is applied then no worries. Others say the crazing can harbour nasty bugaboos therefore it’s not foodsafe.
To test for glaze durabilty in the dishwasher you can run a trial of 2 pieces. keep one in the dishwasher and leave it go through about 40 cycles then compare to the unwashed piece for any loss of gloss or colour. Another way is to simmer a piece in a 5% soda ash solution for 6 hours in a stainless pot, then rinsed, dried and compared to untreated piece.
Really the best tests would be to use the pots for a few months in your own home and see how they stand-up.
Good luck at your Farmers Market (good weather this weekend on the west coast)
edit: some of the people working at SPS know their stuff, others not so much. Tacoma Clay Art makes decent clay bodies and the owner there is very knowledgeable.
Thanks O.Lady but all I want to do is replace the clay handles on my cheese board / trays with bamboo. I don’t need to bend it like the cane handles. 2 short lugs for each side then lash the bamboo on with binding cane.