Firing and Shrinkage

[Daniel Green]

Hi, I have made a series of small tableware slip castings and I want to experiment with variations on size (smaller verisons) without having to make new moulds up.

I was thinking I could just adjust the firing process to increase shrinkage, do you know if this will work and how by how much could I manage to shrink a casting? Would it mean firing for longer, and higher temperatures and at a slower firing ramp (or a combination of these)?

Also, would adjusting the makeup of the slip also help shrinkage (ie. more water)?

Any feedback appreciated.
Dan

[Andy Clift]

I was thinking I could just adjust the firing process to increase shrinkage, do you know if this will work and how by how much could I manage to shrink a casting?

You are making tableware so the casting slip really should only be fired to its maturing temperature, anything hotter will start to melt it. Although you could use low-fire casting slip and low-fire glazes for one set and mid-range or high-fire casting slip for another set. You would just have to find casting slips with very different shrinkage rates. This set-up will probably be more work though than just making a smaller mold set.

Would it mean firing for longer, and higher temperatures and at a slower firing ramp

Speed of firing has really no effect on shrinkage.

Also, would adjusting the makeup of the slip also help shrinkage (ie. more water)?

Adding more water would cause the ware to be thinner, it would shrink a bit more but it will most likely crack during the drying and firing process.

I completely understand wanting to take some shortcuts, but typically in ceramics shortcuts don't work, or they just take longer in the long run, my recommendation is to just make a second set of molds that are a different size.

[Daniel Green]

Thanks for your reply Andy, I had a feeling this was going to be the case :P
My main concern is the cost of getting new prototypes made (very expensive) and the time it takes to make new mould sets (each are complex three piece moulds and Im making 10 of each).
Well... back to the drawing board!
If you do have any ideas in the meantime, please drop me a note. It would make a huge difference to ease of production,
regards,
Dan