Do you have firebrick?

Building a groundhog is on my bucket list but firing them is such an insane amount of work that I should probably opt for something smaller.

Heh...as an A&A customer I was thinking that too, then wondering if someone had found a vulnerability in RevK's code.

Such things exist but are incredibly expensive.

For example, I can't remember where I saw it (some show I saw like 20 years ago), but some really expensive industrial process has a vacuum sealed double walled vessel. Basically, a Hydroflask shaped like an oven. It had low thermal conductivity, but the hard part was keeping the thermal expansion from cracking it open.

Remember those little white tiles on the outside of NASA's Space Shuttle?

I don't have W/mK data...but cost and durability might be issues.

Silica aerogels have a pressure-dependent thermal conductivity, and at atmospheric pressure it is approximately 0.03 W/mK – there are pictures of them on bunsen burners with ice-cream or flowers on top. Their "naked" density is on the order of 2 kg/m^3 -- comparable to that of air (~1 kg/m^3), as they are, unsurprisingly, mostly air. The melting point is around 1400 K (either not as high or comparable to the "2200 degrees" mentioned in the article – I'm unsure if that's ºC or ºF).

The down side, of course, is the cost: all the "real" providers I can find have a price of "contact us", and on Amazon a ~35x25 cm^2 panel goes for about €30, in comparison to somewhere between half and a tenth the price for bricks or other forms of insulation...

[1] https://en.wikipedia.org/wiki/Aerogel#Silica_aerogel

>If someone can invent a high temperature insulator that is as good as low temperature insulators, then that could translate to lots of saved energy.

I'm really not sure these are up to the task, but you can make them thicker as they are cheap.

Here's a carbon foam process from AvE [1] [2]

Also, if you only need one time use, there's StarLite [3] Which is a composite of Elmers Glue (PVA), Baking Soda and Corn Starch. Commercially, intumescent[4] materials are available that are essentially a one time use fire repellant paint.

[1] https://www.youtube.com/watch?v=Wex_yKfrTo4

[2] https://www.youtube.com/watch?v=L-qOIO6IQWk

[3] https://www.youtube.com/watch?v=0IbWampaEcM

[4] https://en.wikipedia.org/wiki/Intumescent

I've built a few coal forges. My insulating material tends to be a mixture of 2:3 plaster of paris to fine quartz sand. I bought the sand from a pool/aquarium supply store.

It holds up well to the heat, but not so much the rain. I tend to remake it every 1-2 years. When it's time I crush up the old bits into a new mix so it doesn't take much more material.

If you want to avoid them for health reasons, there are types of ceramic fiber that can be dissolved by the body (water + mild acid), but they will take slightly less heat, and you'd have to protect them from, obviously, water and acids.

edit: I'd still suggest to enclose them and wear a mask while working with them, but IMHO them being dissolvable makes them much less scary, at least to me.