Kiln firing metal clay, at times, requires that curved or shaped piece be supported during the firing process to help them retain their shape. Metal clay, at times may slump, flatten or warp as gravity acts on the metal while it sinters. As a precaution to this problem a supporting medium is used to ensure successful firing. There are a variety of materials a metal clay artist may use: a fiber blanket, alumina hydrate or vermiculite. My substance of choice is vermiculite. Below I will summarize the three major firing supports.
Disclaimer: Below is information I have gleaned from searches. In these search I have found that I wasn't able to find all the information I was looking for. I tried hard to find reliable documentation to back opinions and ideas I have heard (this was not as easy as I had thought!). I am including the information to get you thinking and perhaps it may help you make your own choice and possibly encourage you to handle your materials more safely. I encourage you to do your own research and share your thoughts.
1. Fiber blanket - is a blanket made of spun fibers of ceramics it is a lot like cotton balls. The problem is that in handling fiber blankets you can dislodge particles of fibers that will then be inhaled and damage your lungs. An MSDS (Material Safety Data Sheet-ceramic fiber) says in bold letters at the beginning of the hazards section POSSIBLE CANCER HAZARD CAUSED BY INHALATION. I did found one vendor which clearly stated warning to purchasers stating "These useful and versatile products do have one drawback, breathing the free fibers presents a health hazard. Always use an OSHA approved respirator whenever this material is being used"
I remember a class I took where the instructor was haphazardly pulling apart her fiber blanket with her bare hands in the middle of class. Hmmm....you want to stop and think. To be honest, I bought a fiber blanket, didn't know how to handle it - it didn't come with any instructions or warnings !!!! Now I know better and I have my mask when I use it and I try to handle it carefully - but I keep it's use to a minimun.
2. Alumina Hydrate - a powdered substance that has been used in ceramic industry for glazes. It is used in a dish by metal clay artists to support the clay. It is a lung irritant. Again, INHALATION is a big concern. The MSDS (Materials Safety Data Sheet- Alumina Hydrate) warns against inhaling the powder.I tried to search out additional data regarding the effects of inhalation. One resource I found was the Canadian Centre for Occupational Health and Safety that had a section on aluminum powders. It is used by many as it has fine particles. BUT with fine particles you have a lot of inhalable material. I have never used Alumina hydrate and never will.
3. Vermiculite: Vermiculite is a naturally occuring material that is used for insulation and as a hydroponic material. This one is my choice of support for my metal clay. It is easy to find - I buy mine at a local garden cente and it works well. I just place it in a crucible or unglazed terracotta dish (sometimes these can crack) and place my metal clay pieces on the vermiculite and fire. It can scratch your pieces and if you are creating a mirror or smooth finish it may damage it.
Regarding safety - I have heard many people say it is safe BUT - the MSDS on Vermicultie recommend that you don't inhale the stuff either. No real surprise here. I still pick this for my choice as well the particles are bigger although there are small bits as well, I try not to disturb the mixture.
One last thing to note about vermiculite and its safety. A student of mine raised concerns about asbestos and vermiculite so I a little digging. There was a mine, specifically in Libby Montana, that happened to have naturally occuring asbestos in the vermiculite.(I would love to go into a long geological explanation here since I was raised by a geologist but I will spare you the details). This mine produced large amounts of vermiculite and a large portion of that that went into insulation it was obviously a big problem. In the 90's the mine was closed. The Libby Mine vermiculite has been off the market for a while now. Vermiculite is now tested for asbestos.
No Support: One last thought...if all this talk has scared you off of using any support. Think about how gravity will affect your piece and try and place it so that it has the least effect. For example if you are firing a domed piece - place it so that the domed part is upwards and the edges to the piece opposite the dome are on your firing surface, the edges will create a drag and will help keep its shape. If you lay it so that the dome is on the firing surface gravity will be more likely to pull on the piece and it may flatten. This may help.
Sadly, I can't find much more information about this at this time. Hopefully this will get you thinking for yourself. If you have any thoughts and opinions please share.