Min

@davidh4976, I was thinking more about the 450F temp and crazing. Is the ASTM554 test still valid if the body cracks but the glaze doesn't?

Pinched your fingers yet? (yup, I've done that a few times )

My guess would be it's because of different reflected indexes between the glazed and unglazed surfaces while in the microwave.

I question the testing using the same piece for each subsequent heat increment as each cycle will stress/shock the glaze. I know that's the point but I agree with Hansen's take on this. I sometimes go 300F/ice water, then 315F/ice water then 325F/ice water. If a glaze doesn't stress craze from this I don't think it will with everyday normal (sane) usage. 450F to ice water IMHO is far too extreme! We are after all making ceramics, not metal bakeware. It does seem logical to soak the test piece for 12 hours after a reasonable stress/shock testing though. edit: In the past I did try the boiling water / ice water testing, had a glaze that passed it so went ahead and glazed a load of pots with it which later went on to craze. I'm sticking with the oven/ice water test, greater difference in temp therefore a harsher test, either the one above or just 3 sets of samples at 315F.

Without going into too specific details to follow the ASTM... for cups and mugs put a cup of cool water in the corner of a microwave then with the empty test sample mug run it for 2 minutes at full power. Handle temp must not exceed 60C/140F and any of the other surfaces 121C/250F. Before commencing test the test pieces must be submerged in room temp water for 12 hours and also pass the ASTM 325F oven to water test. (which I don't have) Slightly different times and temps for plates etc.

I wouldn’t use either of those glazes if you are new to glaze chemistry as it would be necessary to remove some of the other fluxes in order to add zinc. If you just added zinc then the glaze would have more flux than needed to create a balanced glaze. Result would be an excessively fluid glaze. Glazes high in magnesium tend to have a more muted response to colourants than those without high levels of it and blues from cobalt lean more towards purple tones. Yes, zircopax will give more of an opaque glaze, with or without stains. Starting at around 5% addition will show, 10% will be opaque on most bodies. A green stain that contains chrome when put in a glaze containing zinc will most often give an unpleasant brown. Your second link didn’t work for me. Welcome to the forum.

Neoprene disc stuck to the wheelhead with a little water for the vast majority of my pots, no clay wads. For bowls and platters wider than the wheelhead I use a large batt that I glued high density upholstery foam to and put that on a damp Xiem BatMate knockoff. Never felt the need for a Giffin Grip.

In a nutshell to pass the ASTM for "Microwave Safe for Reheating" there is mention of "no metallic surfaces" but no threshold limit for porosity or absorption of the claybody. Ware must show no degradation after the test nor exceed specific temperatures after a set amount of time at full power. I would hazard a guess that those discoloured areas on the plate have let moisture into the body which resulted in the plate getting hot when your son removed it from the microwave. Agree with you, that is one well fitting glaze to stand up for catering use then your home use for 20 years.

You can for sure use wadding in an electric kiln to raise pots off a shelf or level shelves but I wouldn't use it on a glazed surface. If you stack plates with wadding between unglazed areas on the plates chances are the plates will warp and deform at mid or highfire. I don't know the price of setters compared to kiln shelves where you are but another option is to purchase small round kiln shelves and use those with short posts for setters. If you try this be careful to line up posts to prevent cracking a kiln shelf when you stack them, make sure the posts line up. When wadding is used in an atmospheric firing such as wood, salt or soda the vapours from the gasses in the kiln flash the pots but the areas where wadding is will resist the flashing.

Laguna has a chart showing all their claybodies and what they are recommended for, including throwing and handbuilding large. https://www.lagunaclay.com/_files/ugd/e5330f_25040094ad64486892fa46e3a38e608f.pdf

One to look at might be this one for the blue, using the pot spar and cobalt carb you have (increase the cobalt carb to approx 1.60) for the blue glaze, not many ingredients in it and it has a good history. I'ld give it a try with the grolleg you already have. I would also try it with an increase the red iron oxide to tone down the blue a bit and maybe 1 - 1 1/2% manganese dioxide if you have some. For the rim and foot I'ld start with a test of a simple 50:50 mix of your wood ash + your local red clay and see what it does. I agree with Mark, the pot you posted looks more like a salt fired pot rather than soda.

Ouch! I've found that most of the time prices for raw materials are less expensive in the US but my local place has tin listed at $59.10 (Canadian dollars) for 500 grams which puts it at $43 US dollars for just over a pound.

So a breaking blue glaze over a white slip over an iron bearing stoneware body? Maybe with an ash type glaze on the foot and rim?

Tacoma Clay Art Center carries Corelite shelves. You probably already know this but if you order anything from Seattle Pottery Supply I would strongly suggest asking for the current price, not prices shown on their website.

Which effect are you looking to get? Everything or rim and foot or ?