Min

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.

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.

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

I've used dried seaweed, banana peels and copper carbonate mostly so not a huge help with suggestions. Link below with more suggestions, it is behind a paywall but you can access 3 free articles a month. I copy/pasted the chart and snippet from that link below also. If you use ferric chloride wear protective gear while applying it and obviously avoid fumes from the pit firing.
https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/A-Pit-Firing-Color-Palette

 

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. 

I've used dried seaweed, banana peels and copper carbonate mostly so not a huge help with suggestions. Link below with more suggestions, it is behind a paywall but you can access 3 free articles a month. I copy/pasted the chart and snippet from that link below also. If you use ferric chloride wear protective gear while applying it and obviously avoid fumes from the pit firing.
https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/A-Pit-Firing-Color-Palette

 

Underfired glazes can craze for sure.
I always stress test glazes for fit before putting them into production. 

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.

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. 

I have similar, but have also used tupperware boxes, cardboard boxes lined with large plastic bin bags, and fruit boxes, also li Ed.  Anything that is larger "enough".
 

Min - Rather than use roof flashing I use thin plastic sheeting. (I was working at a large corporation, years ago, and they were transiting away from thin plastic sheets, to protect desk tops, to a formica desk top, so I collected lots of thin plastic sheets.)
While the plastic sheets are nice, when I mold round forms, (as opposed to a square pouring box) I sometimes forget that they need to be held down, at the bottom of the shape, as well as strapped together, at the top of the shape, to prevent the whole thing lifting up. (and covering my shoes and pants with a thin layer of plaster splatter.)

While I’m one of the first people who will usually say it’s a bad idea to sell crazed functional ware, why are concerned about crazing on sculptural work? Sculptures don’t have the same durability demands placed on them as dishes. They might have their own durability demands if they’re outside, but I digress.

I spoke with my supplier last week, and they said that Plainsman has a 4 year supply IF no one hoards it. Mike at Ceramics Canada has said he won’t sell more than a 50 lb bag at a time unless you’ve been making your own clay body with it. 
So please don’t panic buy. Give everyone ample time to reformulate behind the scenes while still using their current stock.

This thread is a continuation of this one started by @Mark C. regarding the addition of another K2O feldspar to replace Custer: Vardhman feldspar from India.
Chart comparing Vardhman with other potassium feldspars plus the analysis I received from Vardhman.

 
 
 

Great question, and as you see, there are reasons for different answers.  I dip all of my test tiles, so the are the same on both sides. Then I use small shot glass type testers to see if the glazes interact with each other in weird ways. . . . why the shot glasses?  Easy to throw off the hump, simulate a pot form and allows me to test inside and outside combinations. Finally if there are any weird interactions from different glazes that may cause the pot to crack, craze, shever or anything else, the shot glass shape is more stable and will keep the glaze from doing too much to the kiln. Lots of them can be fired as extras in any load.
 
best,
Pres

Soldate 60 is great stuff. It's a cone 10 clay but for sculptural firing to midrange would work too. I've known people use it for Raku firing also.

Good find Peter.
When I look at the analysis for both there are other differences too though. Definitely a fragile mechanism to get the green. Reitz Green with Gillespie doesn't need the lithium carb and dolomite additions when I played around with altering the recipe, the silica is a tiny bit higher in the Gillespie version but given the differences in the Floating Blue recipe that might or might not be consequential, I don't know. 
A simple addition of iron with a reduction of cobalt would be simple to test Hansen's theory if it translates to Reitz Green also. I suppose RIO would be the logical material to add as long as speckles don't become an issue. Or black iron oxide might be better if you have it. 

Any relevance?
https://digitalfire.com/material/gerstley+borate
... at the bottom of the page, discussing substituting  Gillespie Borate  for Gerstley Borate ...
Clearly, the Floating Blue itself is firing greener than usual. And the Gillespie Borate version is much bluer. You may be used to something in between these two. The green tones could likely be restored by a reduction in the cobalt and increase in the iron oxide.

It might just be as simple as thinning down the underglaze with water and applying 2-3 coats to get the opacity of colour you are looking for. If underglaze is applied too thick it can definitely lift from the clay. 
What works for me is putting the first coat on fairly thin then when that layer is dry I apply another 2 coats to get an opaque coverage. I mostly use Spectrum underglazes but have also used Crysanthos, Velvets, Speedball, LUGs and homemade underglazes.  The only time I don't water an underglaze down is when doing an underglaze transfer.
 

It might just be as simple as thinning down the underglaze with water and applying 2-3 coats to get the opacity of colour you are looking for. If underglaze is applied too thick it can definitely lift from the clay. 
What works for me is putting the first coat on fairly thin then when that layer is dry I apply another 2 coats to get an opaque coverage. I mostly use Spectrum underglazes but have also used Crysanthos, Velvets, Speedball, LUGs and homemade underglazes.  The only time I don't water an underglaze down is when doing an underglaze transfer.
 

If the SG was 1.36 to start with but seems thinner now it's probably just because some of the solubles in the glaze deflocculating it.

Hi Steph and welcome to the forum.
Linda Arbuckle pdf with a good recipe on page 4. What is helpful from this pdf also is the troubleshooting tips on page 3. 
https://www.lindaarbuckle.com/handouts/slips-and-engobes.pdf
Which brands and colours are burning off?