Posted by bpfink on July 15, 19100 at 07:03:04:
In Reply to: Re: Follow-up Question posted by ruth on July 14, 19100 at 10:29:17:
Very good points Ruth.
Any final moisture that can convert from the chemical combination will be able to become steam and that is what can blow the metal back out. Think of it as the principal of the steam engine and the molten metal is the piston.
I make my molds from small sizes like 100 lb or so and on up to 2000 plus lbs each (getting to be like 42 inch dia. and up to 6 ft tall. All are always wrapped around with a cage of 1" mesh chicken wire or in some cases heavier wire like old fencing with a 2 x 3 inch mesh. The wire is put into the circular box before pouring the investment around the piece unless it has to be added and then re smeared with investment to cover and lock it. Seldom do it that way but it does work. These molds can be handled without problems by just wrapping chains around them to turn them over and load them in the kiln. Same after the burnout when they are much lighter now (one of these will have a good 60 plus gallons of water removed) but still have to be turned back over.
Then they are packed into a sand pit for pouring to counter any pressures and close any near holes or weak spots. A must for a pour that has a depth of several feet.
Since the space around a piece is already tight and the addition of more plaster means much more weight I push the limits of it to a fraction of an inch (as opposed to the 2 inches you mentioned).
This can save me another 300 to 400 lb sometimes.
I mark that spot with paint before going into the kiln using a non burnable paint and when it comes out I paste a small patch of ceramic cloth to the side of the mold with a squirt of Elmer's type glue (a water based milk glue but nearly any will work).
Then when it is packed in the sand the patch becomes specifically tamped.
If you would like to see the type size work check out this site and if you would like to see the molds used coming out of the kiln post me an email directly and I'll send it out.
Always love to help those in the teaching phase.
Burnouts like this take me from 5 to 7 days and are done in a firebrick lined kiln with a household type furnace oil burner using .75 gallon per hour. No other monitors or controls (used them in the past but found them redundant). By product of this is that it also heats the studio while in use. It just runs straight out and will take it to 900 F max. It is a down draft kiln so any wax fumes will become more fuel as it drips out. I consider 700 F the minimum temperature with a good soaking time period and it is done when it smells right.
Yes, heard it right, been doing this for nearly 40 yrs now and the old nose is still my best gage. I shut off the burner and wait about 4 minutes, go to the top of the stack, or just open the side top kiln hatch door and waif a little hot air my way and check the sniff. Any acrid smell is too much, anything but hot clean is still not burned out.
Know you can not teach it this way but that is my story and I'm sticking to it.