The following question was submitted by Cary May 3, 2010

hi- i am planning to make a ceramic mold. first, is wax an acceptable medium to use for the original part that is to be copied ?
second, once the part is taken from the mold and finished i wish to attach it to a dowel end, say like a top of a bottle stopper, fitting over the wood dowel. I am concerned about being able to keep a consistent wall thickness so the ceramic parts fit with the parts they are to connecting to, the dowel, etc.,(shrinkage, build-up,etc.)

Hi Cary.

I will try to address your questions and concerns. If I fail, let me know and I will try again.

First of all, yes. Wax is a perfectly good medium, especially if it is a harder wax. Keep in mind however that it most likely can only be used one time as a 'master'. The heat from the plaster as it matures and any undercuts can destroy the original sculpture to some degree.

Next. I can appreciate your concern on wanting consistent wall thickness to the ceramic parts that fit over non-ceramic. This can only be estimated. You could make the opening/fitting in the original before you make the mold; however again you would only have an estimate. There are many variables at play.

If you read some of my articles on slip and clay you will see that slip shrinks as it dries. The amount of shrinkage depends upon the clay body and the formula for the slip. It also depends upon the temperature the item is fired. Higher temps equal higher rate of shrinkage. White earthenware normally shrinks from 8-10 %.

The thickness of the walls will depend upon how long you leave the slip in the mold cavity as well as the condition of the slip. These are all variables that you can learn as you work with your mold and slip. Each mold has its own personality.

The best thing I can suggest is that you make a conservative estimate during the cleaning process of the greenware. You can always make the opening larger after the firing so that your attachment will fit snugly. After you complete a few, you will become expert at gauging size needed.

If you are trying to build a wall as part of the mold cavity (part of the master), then you might want to cast some items in the slip you will be using and do some test firings at various temperatures to see how much shrinkage you will be facing with your chosen slip. Then, you will have a fair idea as to how much you will need to incorporate in the wax master.

Hope these tips will be helpful to you. Again, let me know if you still have questions.

Thank you for visiting Ceramics Made Easy.