Post by smarty on Mar 17, 2013 15:46:45 GMT -8
For photos of this process see here:
www.box.com/s/stnkprr81t1z1vfswg0t
So here is how to make bits of rocket stoves like risers etc. that should be more resistant to heat than their metal counterparts - and insulating into the bargain.
The materials I used were porcelain slip that fires at mid to high temp, grog, and fiberglass.
You start off by making a cardboard former stuck together with tape. Whatever size or shape you like bearing in mind you have to fit it into whatever you use to fire it. (You don't need a kiln just an oil drum lined with ceramic fibre powered by a propane torch of the kind used by flat roofers.)
You coat this form in molten wax that you brush on and then smooth by playing a flame over it.
Next you need to degrease the wax. there was once a product called johnson's floor wax I'm not sure that you can get it anymore, if you can't find anything like this you'll need to google 'pattern wash' Remet do one or maybe get inventive. The basic thing is to treat the wax with something that will make a water based clay slip adhere to the surface.
Then having dried your pattern by placing it in front of a cooling fan, you get your bucket of slip and you dip or pour slip all over the cardboard form. It's easiest to have a shallow trough or something to do this in. Then when you're done you tip the slip back in its bucket.
With the form dripping in freshly poured slip you then transfer it to a suitably sized tray of grog. You sprinkle the grog all over it and pat down any drably bits and then when it is completely covered you stand it back in front of the air circulating fan. You then have to let it dry out completely before pouring more slip over it and coating again with grog.
You do this a total of four times drying between each coat.
After the forth coat has dried you place it in your oil drum lined with ceramic fibre and just whack the propane torch on full blast in the bottom hole of the drum until all the cardboard has burned away. Leave the whole thing in there and continue to heat for half an hour or so to turn the thing to ceramic. Then turn off the torch and once the thing has cooled down enough to get out the furnace and handle you can start brushing on a back up coat.
To do this apply a brushed on layer of slip then while it is wet dip a few chunks of coarse chopped strand mat into the slip - best to use another bucket than the one the slip comes in to avoid contaminating it with glass fibre - and then lay this onto the form by brushing or stippling it on. once the thing is covered in fibre glass you can dry this out with the torch. You repeat this process to get a double layer. the resulting form will be about 10mm/or 3/8" thick or so.
You pop this back in the furnace and heat again at full blast for another 30min or so.
That's it. Because this shell is so full of air it is quite insulating and virtually immune to thermal shock. As long as your slip is reasonably high firing it will just get stronger the hotter it gets. It can be used for any high temp components prone to burn out.
The hardness of the stuff I've been playing about with is such that I think it might very well work in place of cartable concrete for forming the main stove body.
There have been slight issues with dimensional stability with these materials compared to the usual ceramic shell that I used in my foundry, and the initial shell before backing up is frighteningly fragile but once backed up it is solid as a rock.
You will find that the clay starts to crack and peel a bit on the cardboard. This didn't seem to affect the inner surface too much and once fired any cracks that do occur can just be fixed at the back up stage.
The process also has the advantage of being gloriously mucky whic should keep all the cob builders happy. Anyway have a play with it.
www.box.com/s/stnkprr81t1z1vfswg0t
So here is how to make bits of rocket stoves like risers etc. that should be more resistant to heat than their metal counterparts - and insulating into the bargain.
The materials I used were porcelain slip that fires at mid to high temp, grog, and fiberglass.
You start off by making a cardboard former stuck together with tape. Whatever size or shape you like bearing in mind you have to fit it into whatever you use to fire it. (You don't need a kiln just an oil drum lined with ceramic fibre powered by a propane torch of the kind used by flat roofers.)
You coat this form in molten wax that you brush on and then smooth by playing a flame over it.
Next you need to degrease the wax. there was once a product called johnson's floor wax I'm not sure that you can get it anymore, if you can't find anything like this you'll need to google 'pattern wash' Remet do one or maybe get inventive. The basic thing is to treat the wax with something that will make a water based clay slip adhere to the surface.
Then having dried your pattern by placing it in front of a cooling fan, you get your bucket of slip and you dip or pour slip all over the cardboard form. It's easiest to have a shallow trough or something to do this in. Then when you're done you tip the slip back in its bucket.
With the form dripping in freshly poured slip you then transfer it to a suitably sized tray of grog. You sprinkle the grog all over it and pat down any drably bits and then when it is completely covered you stand it back in front of the air circulating fan. You then have to let it dry out completely before pouring more slip over it and coating again with grog.
You do this a total of four times drying between each coat.
After the forth coat has dried you place it in your oil drum lined with ceramic fibre and just whack the propane torch on full blast in the bottom hole of the drum until all the cardboard has burned away. Leave the whole thing in there and continue to heat for half an hour or so to turn the thing to ceramic. Then turn off the torch and once the thing has cooled down enough to get out the furnace and handle you can start brushing on a back up coat.
To do this apply a brushed on layer of slip then while it is wet dip a few chunks of coarse chopped strand mat into the slip - best to use another bucket than the one the slip comes in to avoid contaminating it with glass fibre - and then lay this onto the form by brushing or stippling it on. once the thing is covered in fibre glass you can dry this out with the torch. You repeat this process to get a double layer. the resulting form will be about 10mm/or 3/8" thick or so.
You pop this back in the furnace and heat again at full blast for another 30min or so.
That's it. Because this shell is so full of air it is quite insulating and virtually immune to thermal shock. As long as your slip is reasonably high firing it will just get stronger the hotter it gets. It can be used for any high temp components prone to burn out.
The hardness of the stuff I've been playing about with is such that I think it might very well work in place of cartable concrete for forming the main stove body.
There have been slight issues with dimensional stability with these materials compared to the usual ceramic shell that I used in my foundry, and the initial shell before backing up is frighteningly fragile but once backed up it is solid as a rock.
You will find that the clay starts to crack and peel a bit on the cardboard. This didn't seem to affect the inner surface too much and once fired any cracks that do occur can just be fixed at the back up stage.
The process also has the advantage of being gloriously mucky whic should keep all the cob builders happy. Anyway have a play with it.