Have you had small rough rock, or round rocks that gave you concern as how to best to saw them or want to be able to slab the rough completely? In the process of sawing round rocks of hard agates, such as Brazil agate nodules or other agate nodules, the saw blade can get in a bind, and consequently dish the blade if the blade is allowed to start to drift. One option for safely and completely sawing this type of rough is to make a Plaster of Paris mold. Molds can be easily made using a wax coated carton, (milk, buttermilk, orange juice cartons) for the container or make a wooden mold form shaped like a loaf of bread. These containers are alternately filled with Plaster of Paris, (P.O.P.) between each rock at least 3/8" thick and on all sides of the carton. Follow the suggested mixing directions on the P.O.P. container. (Two parts P.O.P. to one part water.) P.O.P. is available in small containers up to very large sacks at your local hardware stores. Prepare all necessary items so the filling of the mold will take as little time as possible. The setting up time is approximately 30 minutes, but drying will take longer. You'll need: wax coated carton or wooden mold, Plaster of Paris, bowl or bucket for mixing the P.O.P., water, container for measuring the P.O.P.(a large plastic cottage cheese carton with wide open mouth gives quick easy measurements), stirring stick or spoon for mixing the P.O.P. and water, and rocks. As you are placing the rocks in the container, remember to orient any rock that has a special pattern. The mold will be slabbed or sliced like a loaf of bread. Leave a portion at the top of the mold for filling with 2-3" of P.O.P. and this end will be clamped in the vice. (When the mold drys, I like to write in pencil, the type of material in the mold and you know this is the clamping end.) As you are filling the mold, tap the mold on the table or on the floor several times to help settle the P.O.P. and fill in any possible gaps. Let the mold setup for several days or a week to be sure it is dry and then the P.O.P. mold is ready for slabbing. If the mold is in a wax carton, slab right through the carton with no problem. When the slab is finished sawing, the Plaster of Paris will easily break off from the slab. If a reusable wooden mold is used, after drying time, just remove the mold and the P.O.P. mold will hold together. Every square inch of the rough can be slabbed and the Plaster of Paris guides the saw blade in the proper course without allowing the blade to shift and cause problems with hard agates, etc.. Instead of slabbing one small slab at a time, you may be slabbing 2 or 3 at a time, depending on the size of your rough and P.O.P. mold. On large molds, we occasionally have one end of a round rock pull out, due it being so round and smooth and not enough pressure on what is left to hold the rock, but this does not happen often. We have found no problem with the grinding of the small amount of P.O.P. in the saw oil.
Another option for slabbing round or small rocks is to glue a heel or half of a nodule, etc., onto a board with white milk glue, let dry, and slab. Clamp the board in the vise and crank the rock over to the blade as far as it will go, then crank one turn back, and continuing to crank back in multiple's of six turns (approximate 1/4" thick slabs) and then set the vise ready to start the slab. You will be able to take advantage of all the material and end up with just a sliver of material left on the board to throw away. (Remember, this board is coated with oil and will not be usable again for gluing another rock.)