Sawdust or foam beads to insulate?

[Orangejoe]

I have had a score of different forge and melting furnace builds. Kaowool with kol30 liner, homemade grog and clay, castable with perlite and a hotface...

I know it goes against the conventional wisdom on here, but I found i preferred a commercial refractory mixed with perlite at a 1 to 4 by volume ratio with a thin refractory hotface, that is with one exception- the perlite in the insulation layer slowly begins to flux through after one or two dozen copper melts. I rushed when i made this, and should have read more.

 

I have been using the Wayback Machine to read some old archives on alloyavenue/ backyardmetalcasting and a bit here and am interested using either sawdust or small (1/8" and below) styrofoam beads to form air pockets in my insulating layer on my new project.

I see a lot of different opinions on which is better and why, as well as whether finer or slightly more coarse is better. This will be a 2 inch layer behind a 1/16 to 1/8 inch hotface of the same high alumina castable refractory. Thanks for the help guys.

[Charles R. Stevens]

Personally I would be concerned about out gassing. So I would stay away from the plastic. 
Glenn and others sell small sections of koolwool and rigid user for reasonable prices, and an inch or so of the ceramic over your home brew and faced buy say a 1/4” of castable might be a good compromise. 

[Frosty]

Sawdust, grog and fire clay is an insulating refractory recipe from 30-40 years ago. I heard about it from potters and ceramics folk using it in their kilns. I was looking to build a forge and was unaware of commercial refractories. I thought about using styrofoam beads when bean bag chairs were filled with really tiny beads, the kind that are forever hiding in any house a beanbag chair sprung a leak. 

I tried styrofoam beads in a 1" x 2" x 2" test coupon. As well as a sawdust coupon. The sawdust coupon was weak and crumbly, it needs to be pressed to work like hard firebrick is. 

The styrofoam coupon didn't quit smoking. I bisque fired it thinking that would burn the beads out. Nope. Then I put a blower to the lump charcoal I was firing with and brought it to yellow heat. Nope the coupon smoked when it cooled below red. I then fired it with an oxy acet rosebud till it vitrified and started to slump. Still smoked.

That was probably 40 years ago and led me to ask a few questions and purchased commercial refractory a 100lb sack of fire clay dost more than a 50lb box of 3,000f phosphate bonded refractory.

Why do I spec phosphate bonded? Because no matter how you modify or fire, fire clay it's a silica clay and the borax fluxes commonly used forge welding dissolves silicates at welding temps.

My advice is forget about styrofoam beads, it's a lot of trouble to get to work and it's unhealthy. The sawdust admixture will require a  serious press or it'll be weak. That might be okay with you if you can support it but once again a lot of hassle.

I'm not a fan of saying something won't work, without an alternate suggestion or two.

How about using what the commercial companies use to make insulating castable refractories and light weight concrete?  The "bubbles" that increase the insulation value of Kastolite30 are evacuated glass beads and they're silly cheap at masonry supplies and concrete batch plants. 

I suggest you make test coupons to determine how much to use. There is a point where the melting glass in the refractory will so weaken the liner it'll just slump into a puddle so experiment. I suggest you test to at minimum a couple hundred degrees hotter than you run your melter.

Frosty The Lucky.