Broken Ceramic Mold (Made of Plaster)

How to Repair a plaster mold

When casting molds, we are all careful because we know to handle them gently; however it is inevitable that one will eventually break. When that happens, all is not lost unless the mold has shattered into a zillion pieces. If the parts are of manageable size, however, they can be reassembled much like a puzzle and once again be castable.

There are so many molds that are no longer on the market - and those that are, are so costly to replace and ship - that to me it is worth the effort to repair them when possible. I will discuss several methods of repair and offer tips to help you save that mold which may not be replaceable.

A lot of people have a lot of ideas on how to repair plaster molds. I am only including the best ones based on my many years of experience. If you hear of other methods, by all means feel free to try them.

Elmer's Glue

Repairing with Elmer's white glue. Yes, there are other white glues out there, but not all are Elmer's and I highly recommend using the name brand.

To repair a simple break where the edges are relatively clean and they fit together well, you need to take both pieces and dip the broken sides in warm water for 30 seconds to one minute or saturate broken edges with a soaked sponge. The plaster needs to be wet in order for the bond to be secure. If you don't wet the mold, the glue won't hold as well.

Thin a puddle of glue with water to a consistency of buttermilk. It needs to be thin enough to soak into the mold but creamy, not watery. If you do not wet the mold and thin the glue, you will NOT get a tight fit of your pieces and you will end up with a gross seam line.

Use a brush to spread the thinned glue over both broken pieces, however do not put the glue on the edge of the inside/cavity of the mold. Keep back about 1/16 of an inch. Be sure to put your brush in water so the glue won't set-up until you can clean it.

Press the two pieces together and hold firmly. Depending on the size of the mold, use a belt or rubber band to hold the pieces together. Place it around the outside perimeter of the mold.

Quickly wipe off any excess glue that may have seeped inside the cavity. Use a wet sponge and wipe away any glue residue from the inside of the mold or along the inside seam. You may have to rinse the sponge a few times and keep wiping to ensure there is no glue residue left. If you do not clean away all residue, it will hinder your casting process and will create a hardspot on your ware.

Place a piece of paper or plastic wrap across the broken half (covering the cavity and glued inside seam), then fit it together with the non-broken half of the mold and belt or band it together, making sure it is a tight fit and that the broken pieces are properly aligned.

I recommend you let the mold cure for a few days, until it is thoroughly dry. Remove the paper from between the mold pieces. Depending upon the mold, you may want to keep the outer perimeter band on the half that was broken for additional stability, but it should not be necessary. If the repair does not hold, then you might want to repeat the repair process and leave the band on the broken half. If it continues to break during usage, you may need to bridge the repair with a plaster patch on the outside of the mold. (more later on this process)

If a mold has broken in more than two pieces, the process will be the same. You will just need to work on the pieces one at a time and assemble them as a puzzle. The great thing about Elmer's is that you have time to work. The wet mold and the thinned glue with give you time to assemble all your pieces, but be sure to clean up seams of excess glue as you go. You do not want any soaking into the cavity of your mold or it won't release the greenware when you cast it.

If you have chunks missing or holes, it will be necessary to fill them. Depending upon size, smaller areas can easily be repaired by mixing a paste of mold plaster and filling holes from the outside. I use a small amount of clay and press it on the inside cavity to create a thin block to stop any plaster from seeping into the cavity.. Again, the area needs to be 'wet'. Mix your plaster with equal parts (by weight) of cold water. Remember powder/plaster is dry weight and cold water is liquid weight. Take a small portion of the mixed plaster and add a bit more water to thin it and paint a thin layer on the holes so it will soak in and then fill with the proper mix. You will have about 5-10 minutes to play with it and get it smoothed out and shaped. Mold plaster is US#1 pottery plaster and is available at most potters supply stores by the pound.

You can create a 'bridge' on the outside of the mold also, if you desire or feel you need the added strength (especially on a large thin mold like a plate or platter or base). On the outside of the mold, simply saturate a wide area on either side of the repaired break with water and then brush or pour some plaster that has been thinned slightly and build a bridge covering both sides of the break. It needs to be thin enough to soak into the mold, but not so thin as to run out of control. I sometimes run some wide tape around the perimeter of the mold half creating sides for the bridge and cover the whole side with a good layer of the plaster...the tape helps keep it from running over the edges. Be sure to clean your brush or put it in water before the plaster hardens. Allow to harden and dry before casting the mold.

APT II

Another good medium on the market is Apt II Mend-a-Mold. Mend-a-Mold is a ready to use adhesive that will work on wet or damp molds. Simply apply to both sides of the broken pieces, press together and hold a few minutes. Use a wet sponge to wipe away excess from cavity and seam, if it is a two part mold, so the parts don't stick to each other and then put your mold back together and band it until the mold is dry.

Apply pressure while adhesive sets - a rubber band or banding strap may used. After 2-3 hours, your mold is ready to use as long as it is no longer wet. (Not a good idea to cast wet molds, but that is fodder for a different article)

Some people claim that regular Apt II will also mend your mold, however this ceramist has not tried it.