Plinth location - or qualification even. Is it recommended/not recommended to cover a fire brick in refractory material at the base of the foundry to count as a plinth? Or is a plinth specifically a free brick or platform in the base of it? As I understand it the purpose is mostly to allow for the flame to be properly positioned, and that that proper location is to position it so the flame is in contact with the bottom of the crucible. A cold bottom of the crucible might cause the melting metals to simply sink back down and solidify. I've seen plenty of examples of people with the brick in the bottom that use cardboard beneath it to stop the brick from sticking to the crucible, the layer of ash apparently is what's accomplishing that. I have a hard time believing a crucible would pick up that refractory material, or any amount of concrete it's placed on top of for that matter. Is it necessary or recommended to use cardboard if you did prepare the bottom of the foundry by pouring refractory material over the plinth? Or is that only necessary if you're just setting directly on top of a brick? Thank you again!
IMO a plinth is supposed to keep the crucible out of the direct path of the flame, not to expose any part of it to more direct flame contact. It's the the hot furnace wall that should heat up the crucible as evenly as possible from all sides. Direct flame impingement can corrode a crucible badly and reduce its lifespan significantly. Firebrick or refractory, no matter, crucibles will stick to them all if you forget the cardboard. If you're asking if it would be a good idea to actually build into the furnace floor rather than using a loose plinth, I would say no. If your crucible ever does stick to it, you want to be able to get it out of your furnace somehow... Jeff
A loose plinth block lets you try different heights in case there are unforeseen issues with your furnace design and is easily replaceable if it gets damaged from excess cracking. Also you mentioned running off propane gas so there would be no oil leak issues if you had a removable bolted on hatch under the furnace base. This would let you easily replace the floor of the furnace if there was a molten metal spill and also replace the refractory base from wear and tear. Both the gas fired and oil fired furnaces I use have base hatches and the gas fired one even has a fibreglass mould to cast an integral floor/plinth every so often as needed. After the refractory cures it gets grouted in the base of the furnace.
It is the coating of the crucible melting and running under the crucible that causes sticking. Tobo gave correct advice, IMO, regarding the layers of cardboard. Even a couple of layers of cardboard do not guarantee no sticking will occur. But it also matters how hot you are getting the furnace and crucible. If you are melting iron, the sticking issue is much more likely to occur than if you are melting aluminum. I also "grout in" my plinth (like Mark above) to the furnace floor. That way, if sticking occurs, the plinth remains on the furnace floor and the sticky bond between crucible and plinth is overcome with the lifting equipment. In the old days when the plinth was not grouted in, I had the plinth lift out with the crucible---a very awkward situation. I use BluRam refractory for my plinths and use it to grout in the plinth as well. Denis
Further info to add to what Denis and Tobho have said about the crucible sticking to the plinth, I use some 4mm or 3/16" sheet cement cut to an octagon just bigger than the crucible base as a disposable separator between the crucible and the plinth to ensure separation even when the crucible glaze or molten iron slag is trying to glue everything together. The product is called "Fibro" in Australia and these days has cellulose fibres in the cement as opposed to the asbestos used until the early 1990's in Australia.
I have tried, at Mark's recommendation, the sheet cement in the form of the very common cement siding products sold under various names like Hardie Plank. It works well to resist adhesion. But, one thing to be aware of, it seemed to accumulate in the bottom of the furnace as it does not burn out. So, it can/will gradually build up a level of residual cement in the furnace. Denis
As I recall, it was a layer of rock-hard material that I was worried would build up unacceptably if I used the cement board repeatedly. Had it been soft or dusty, I would not have worried. In my case, just using a 1" in diameter "sausage" 18" in length of BluRam pressed into the junction of the plinth and furnace floor solidly fixed the plinth to the floor. That was a permanent solution as the strength of adhesion of crucible to plinth is usually not that great. But, otherwise, it can be enough to have the plinth come out with the crucible. Early-on I had to brush the crucible across the lip of the furnace on an occasion or two to dislodge a stuck plinth---tricky and could lead to spilled metal. That worked to knock the plinth loose, however. Not fun. Denis
I always tilt the crucible to one side with the lifting tongs to break off the plith if it is stuck , one of the reasons my furnace bore is rather big compared to my crucible V/r HT1 p.S. but I have poured with the Plith attached to the crucible, the extra weight is NOT welcome
It could be the type of crucible, extra iron slag on the plinth, some variation in the sheet cement or the final temperature: all I get is a fragile, shiny fused cement coating stuck to the crucible that breaks off easily, I just leave it there and use it a few times until it falls off when the cold crucible is sitting on the concrete floor. I found a local deposit of kaolinite clay so I'm going to experiment with it for plinth coatings and for furnace repair: apparently it converts to mullite at 1050 deg C so it should be similar to coatings like Satanite.
