Hi
I hope someone can help me I have turned out a plate in the shape of a disc plough blade and then cut it square. I have cast a plaster mold in order to make a clay mold. I have raku slip and I was wondering if it was the right clay to use for my slumping molds Do I fire the clay mold once it is dry to 982C and can I use the slip as is or is it neccessary to add silica sand or whatever to the clay
thank you
Ian
raku clay slip
Moderators: Brad Walker, Tony Smith
The raku slip should be fine for casting slumping molds because it is made to handle sudden temperature changes in raku firing. Our heating and cooling rates are much slower than the change from the kiln into the reducing material of the raku firing. Firing the mold to 982C is about 1800F, just slightly cooler than the regular cone 06 suggested for most earthenware clay used for slumping molds. This should make the mold durable. I assume you are making the plaster mold so you can duplicate your slumping mold several times for production use or to sell copies of your mold. If you need only one mold you should be able to bisque your turned clay model and and use it for a slumping mold if it will sit level on the kiln shelf. Remember to make small holes through the mold(s) where the glass will touch the mold last when it settles into the mold in the kiln. This is so much easier to do before the mold is fired! Good luck with your new mold and let us see some pictures when you slump into it.
Best wishes,
Tom in Texas
Best wishes,
Tom in Texas
I wasn't clear about how you produced your original model. Sure enough wood would not work for a slumping mold. As a matter of interest what did you use to seal your wood master when you cast the plaster against it? What species of wood did you use for the original bowl, what diameter and depth was it originally and how large is your square? Inquiring minds need to know.
Best wishes,
Tom in Texas
Best wishes,
Tom in Texas
I turned out the master in Jelutong. It is rubber tree wood and is very close grained and is a soft wood. It is used mainly for carving. I finish it with sanding sealer and then a couple of coats of polyurethane varnish covering it completely. I started with a block made up of 2x 12.5 inch squares and laminated them to give me an overall thickness of 2.25 inches I then turned it out to a depth of 1 inch and turned in a foot ring for the finished dish to stand on. The idea is to be able to use the same mold for a 12inch plate, a 6 in plate and also to use rectangles , ovals and ellipses of different sizes I also thought that when I get to do pot melts I can use this mould to slump them
How slow should one ramp up for firing the raku?
Thank you for your help
Ian
How slow should one ramp up for firing the raku?
Thank you for your help
Ian
150 C (300F) per hour is the rate used by Orton in their cone / temperature equivalents charts. Thanks for the information about your wood shape. It sounds like you will have a nice slumping mold. I like the idea of the integral foot ring to keep the plate flat on the table. I would think you need 4 equally spaced 1/16" (1.5mm) holes through the bottom of the foot ring to allow the air trapped in it as the glass slumps across it to escape. If the mold sits flat on the kiln shelf on the back side of the foot ring cut a small groove from each hole to the outside of the ring to give the air some place to go. There is a chance you might also need vents at the location on the sides where the glass will touch the mold last. Many of the commercial slumping molds I have seen are supported above the kiln shelf by a rim of clay about an inch tall. To get this rim with your wood master you would have to build up behind it with clay to support it about an inch above the surface it rests on when you cast plaster around it to produce your mold. A good example of this can be seen at http://www.crestmolds.com/glassaggers.html
Scroll down to CS157 to see an example. The holes in the supporting rim are to allow the air displaced through the 1/16" vent holes by the slumping glass to escape. Again, let us see some pictures when you get your glass slumped.
Best wishes,
Tom in Texas
Scroll down to CS157 to see an example. The holes in the supporting rim are to allow the air displaced through the 1/16" vent holes by the slumping glass to escape. Again, let us see some pictures when you get your glass slumped.
Best wishes,
Tom in Texas