Once I got into casting, I saw it as a great process to make vessels. Being inspired by the consumption and storing of alcohol I began casting bottles and flasks intended for corks but always pondered the idea of casting threads. I saw it as a slightly impractical solution but wanted to have the technical skill to pull it off. The problem with making them initially is that if you pour plaster inside of a threaded cap, when you unscrew the cap, the plaster would break. Using a low thread count like a growler cap which has one slow thread, one might get away with this simple solution. However, I knew I wanted a higher thread count.
Since plaster breaks when cast inside of a cap, my solution was the change the material of the cap so that it was flexible enough to release. Mold-Star 30 from Smooth-On is a silicone that is made for casting plaster mother molds for. I figured this would be the best solution. I tried casting the silicone inside of the cap to make the negative, and then cast silicone against that to create the silicone positive of the cap. However, the silicone seizes onto itself and ripped when unthreading. I looked for other silicones or urethanes that would work and realized I would run into the same problem despite what mold release I would use.
Once, again I knew I needed to change the material. Casting wax used for burnout molds for aluminum and bronze molds was the key. It was strong enough to maintain its shape but would not adhere to either the cap or the silicone. Before casting the wax negative of the interior threads the cap needs to be blocked in. A tiny coil of clay needs to be ran inside the top part of the cap where the threads stop. Generally in caps there is a ring of space where a gasket goes or where the threads terminate. This would cause an ‘undercut’ that will not unthread. Once treated, the wax is heated up until liquid and poured inside. After it sets it will unthread smoothly. Then the Mold-Star 30 is poured over the wax which will create a silicone positive of the cap. Then plaster can be poured into the silicone to achieve the interior thread mold.
Once the interior thread molds are made, the cap design can be considered. It needs to be thick enough to be strong enough to withstand minor chips from the threads as well as thick enough to realistically be poured. Then I trimmed the cap out to fit over the plaster threads and I cast the second half of the mold, the top of the cap. I drilled a hole in the very top center to pour slip in and cast solid.
The other part of the threads, the male thread, is the easy part. This consists of a four part mold, two release from the sides of the threads and one for the top and one for the bottom. The bottom of this is then cut out and can then go on any desired flask or bottle. Once these pieces are bisqued I thread them, there will be some resistance, to lightly grind them while the ceramic is still relatively soft. Once the pieces thread smoothly they can be fired to maturation. Alumina wax can be used in an atmospheric kiln to help resists ash buildup which would inhibit their function. However, using alumina wax can be unsightly when you have a woodfired piece that is full of color and depth but raw clay threads. Hiding the pieces in the kiln behind larger works helps protect them from ash but lets the threads still receive some atmosphere. The threads will not initially work and if forced it will break. In order to sand this down mix up a slurry of silica carbide powder and water. Brush this into the threads and slowly start to turn them on an off, this is a slow process but eventually the cap will thread all of the way on and then the silica carbide will sand the threads so they work without the cap squeaking which can be quite unpleasant. Once finished wash the threads with water to remove excess silica carbide.
The threads alone are not enough to seal the container. If it is desired to be airtight, silicone must be put in the cap. Once again I use Mold-Star 30 for this. I mix up a small amount and using a syringe, it can be accurately administered into the cap. I fill up the cap until the silicone is at the same level as the last thread. Now when the cap is put on the lip of the flask or bottle will sit slightly into the silicone and make an airtight fit.