PeterH

Throwing out a thought that might simplify this debate. 
Q: what colour response does cobalt give in a high magnesium glaze? (including midrange)
A: shades of purple
If the magnesium wasn't entering the melt as a flux then no purple.
Dave Finkelnburg article on cobalt purples and magnesium.
 

Peter, I'd copied the quote to the notes I was keeping whilst reading up on eutectics; I wanted to credit the source.
I'm not finding it again, almost sure it was from potters.org; sorry I haven't found it, I did look!
"hmmm" on account of several notions it points to (imo):
  Several authors point out that dissimilar particles in a dried slurry behave differently on heating than powder ground up from previously melted/solidified material of same proportions.
  Typical glaze slurry may have several eutectic "things" happening, much more complex than the three legs of a pyramid.
  Liquidation and solidification likely occur over a range of temperature change and takes time (like butter, as others have pointed out), which is probably good.
As for "Why?" does my liner glaze with the talc have less excess Silica and Alumina, per EuCal, than the "same" glaze without talc (MgO) my guess is that untangling the math would answer.
From there, testing could prove.
   

Hi Gail and welcome to the Forum!
In an ideal world we would use a low expansion claybody paired with a well fitting low expansion glaze for ovenware. In reality a lot of people don't do this yet still make ovenware, usually when selling the pots they add a cavet to place the filled pot in a cold oven then turn the oven on along with other advice like using it in the middle of the oven and taking the design into consideration. (even wall thickness, no pooled glazes, no sharp corners etc)
To make a midrange low expansion clay manufacturers use materials like spodumene, pyrophillite and carefully control the silica. Adding these to an existing body is going to alter the maturity temp/cone of the clay. I would suggest asking your supplier which claybody they recommend for ovenware, ask the manufacturer if the supplier isn't sure.
 

Don't some/many commercial "paper" clays use flax fibres, which I assume don't rot.
My experience with paper clay was also long time ago. Non-paper fibres were available but IIRC the minimal order was both expensive and very bulky -- while cheap toilet paper was readily available everywhere.
PS Another option to prevent rot, keep the paper clay dry.
Also see the next post in that thread, which recommends copper carbonate. Which I believe  is sometimes used as a disinfectant in in glazes.

Don't some/many commercial "paper" clays use flax fibres, which I assume don't rot.
My experience with paper clay was also long time ago. Non-paper fibres were available but IIRC the minimal order was both expensive and very bulky -- while cheap toilet paper was readily available everywhere.
PS Another option to prevent rot, keep the paper clay dry.
Also see the next post in that thread, which recommends copper carbonate. Which I believe  is sometimes used as a disinfectant in in glazes.

Kiln Breaker Sizing, Code, Manufactures breaker recommendations - North America electrical practices
Just thought I would post here, these things seem to come up often. Anyway ran across an inspection I did several months ago. The issue was breaker sizing for multiple commercial water heaters, which are very similar to a kiln load in many respects. Anyway long story short, in NorthAmerica  breakers sizings are usually limited to 80% design load or for special continuous use resistance appliances a minimum of 125% of the continuous load but not more than 150%. This was created to keep the operation of the breaker in a typical panel reasonably cool and therefore extend the life and reliability of the breaker. My anNonimized  picture and infrared photo below depicts the unexpected continuous run time of one of a bank of water heaters (over three hours). Anyway, the breaker was sized slightly under 125% of the water heater load  but still at least 100%. The manufacture listed the maximum breaker size as 30 amps. The electrician used a 25 amp, stating it was enough.
Anyway, I think my infrared picture in operation at over 133 degrees f and the fact that it tripped validates the reason for the code and manufactures generally conservatively follow that code. Additionally this code has true time tested proven merit so ignoring it or misunderstanding it can reduce breaker lifespan and elevate the hazard of fire. It is scary to think an inspector would not take the time to learn their craft.
Codes are important, manufactures have insight into the engineering and operation of their equipment. Generally their minimum and maximum recommendations are well researched and compliant with code requirements.
Sizing breakers for kilns seems to be a tricky thing. Sizing them smaller than the manufacture suggests is probably not a good idea as can be plainly seen below. Hopefully this post will be useful for someone searching in the future wondering.
 
 

I don't think that firing within the manufacturer's guidelines guarantees that your form won't slump.
Probably only an issue with forms outside the usually pottery shapes, as previously observed by Hulk.
Think of a sheet of printer paper. Hold it by the short edge and it flops about. Tape the long edges together and it forms quite a strong cylinder. Compound curves are probably even better at resisting deformation.
I suspect that during firing sheets of clay bend easily, unless the bending requires local stretching or compression of the sheet.

I don't think that firing within the manufacturer's guidelines guarantees that your form won't slump.
Probably only an issue with forms outside the usually pottery shapes, as previously observed by Hulk.
Think of a sheet of printer paper. Hold it by the short edge and it flops about. Tape the long edges together and it forms quite a strong cylinder. Compound curves are probably even better at resisting deformation.
I suspect that during firing sheets of clay bend easily, unless the bending requires local stretching or compression of the sheet.

I don't think that firing within the manufacturer's guidelines guarantees that your form won't slump.
Probably only an issue with forms outside the usually pottery shapes, as previously observed by Hulk.
Think of a sheet of printer paper. Hold it by the short edge and it flops about. Tape the long edges together and it forms quite a strong cylinder. Compound curves are probably even better at resisting deformation.
I suspect that during firing sheets of clay bend easily, unless the bending requires local stretching or compression of the sheet.

I don't think that firing within the manufacturer's guidelines guarantees that your form won't slump.
Probably only an issue with forms outside the usually pottery shapes, as previously observed by Hulk.
Think of a sheet of printer paper. Hold it by the short edge and it flops about. Tape the long edges together and it forms quite a strong cylinder. Compound curves are probably even better at resisting deformation.
I suspect that during firing sheets of clay bend easily, unless the bending requires local stretching or compression of the sheet.

Good question!
My guess is the clay softens just enough to overcome the form, where a cylinder or even an open shape - bowl - has enough structure to resist collapse, your four twisted ribbons of clay cannot, so.
Perhaps if the four arms had some arc, and no twists, they would stay upright.
You might try using a prop - a narrow cylinder - to hold the form upright, where the interface isn't glazed and has a bit of kiln wash or somewhat to prevent sticking. Even so, the twisted ribbons may still sag.
Good luck! I'm curious to know how your project comes out.

Assuming the damp box started out with water in it, it could simply be a magnified form of efflorescence of some soluble salt as the water seeped into the pot and then evaporated from it..

I imagine that the relatively small amount of plastic materials in many casting slip doesn't help.
https://digitalfire.com/article/understanding+the+deflocculation+process+in+slip+casting
First consider how casting body dry ingredient recipes differ from plastic bodies. Casting bodies do not need to be nearly as plastic as jiggering, throwing or pressing bodies since they do not require shaping. Plasticity is only needed to impart shrinkage so the piece can pull away from the mold and to give it adequate dry strength. Too much plasticity is actually bad because plastic materials are less permeable to the passage of water through them, the casting process is slower (casting speed is an important factor in the process) and ware can split in molds with undercuts.

Note: Throwing vs Casting - In my experience, to carve holes through thrown porcelain, stoneware and earthenware is different than on clay cast in a mold. The clay of a poured form has less integrity when cut and carved, compared to the aligned clay molecules that have been wheel thrown. Thinly cast forms are more fragile both before and after firing. 

Making a "nice" egg shape might be a little tricky.

If you don't have throwing skills you might try something like this...
You might find refining the shape of the two cups on a "found" egg-shaped object before you join them gives more realistic results. Although I doubt that a chocolate Easter-egg would be strong enough. Maybe an Easter-egg shaped gift box?
If you do have throwing skills there are several egg-throwing videos on YouTube, such as ...

https://youtu.be/uXO8wlCVxLA
https://youtu.be/iymjWjUVskI
https://youtu.be/D8OyAgu-cjI

A marker will just gum up with the clay, so something that brushes on would work best, like food coloring or india ink.

This effect of variation near texture is know as glaze breaking.

A not very helpful link to start you off
 

This effect of variation near texture is know as glaze breaking.

A not very helpful link to start you off
 

Making a "nice" egg shape might be a little tricky.

If you don't have throwing skills you might try something like this...
You might find refining the shape of the two cups on a "found" egg-shaped object before you join them gives more realistic results. Although I doubt that a chocolate Easter-egg would be strong enough. Maybe an Easter-egg shaped gift box?
If you do have throwing skills there are several egg-throwing videos on YouTube, such as ...

https://youtu.be/uXO8wlCVxLA
https://youtu.be/iymjWjUVskI
https://youtu.be/D8OyAgu-cjI

This effect of variation near texture is know as glaze breaking.

A not very helpful link to start you off
 

Off topic, but as you seem to be exploring the boundaries at the moment so you may be interested.
Historically millions of bowls have been made in bisque moulds. Not by casting, but by a skilled-labour intensive slow-wheel process akin to manual "jiggering".
 
Also see https://www.youtube.com/watch?v=2Nef_LuFDkc
Note there is no attempt to mould-form the foot or the upper smooth-section-with-rim. But you get a lot of texture with little per-pot effort, and the use of stamps minimises per-mould effort.
 

You can get a similar look by squeezing a natural sponge in thick slip then firing that. I tried it with different sponges and the rougher sponges with the larger holes did turned out well. I found inexpensive sponges that were a bit rough looking at a dollar store. What worked for me was to get the sponge wet with water first then squeeze the water out really well then squeeze the sponge in a container of slip. I was using porcelain slip made from a pugged clay, nothing but water added to it. 
Link here which showing another members work with it.  https://community.ceramicartsdaily.org/topic/17192-clay-sponge-slip-submerged-sponge/

May be of interest, as it indicate that clay:water ratio and slip fluidly can -- to some extent -- be controlled independently.
 
 

I'ld run it through the finest sieve you have then let the undissolved residue of soda ash dry in the sieve then dump it out and pour very hot water over the residue and add it back to the glaze slurry. (If you try and scrape the undissolved residue out while it's still wet it's harder to get all of it out)

Took the panel off, found a wire on the 3rd element to the switch not connected good, fixed that. Also the top element was broken where it bends in through the brick. I guess my moving was kinda rough.  Going to order all new elements and see what gives, lol.