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Ceramics Pottery Plaster Molds
Plaster Molds

It is interesting to read the legend of man's discovery of plaster in view of the fact that it took place many thousands of years ago. Since that time, the material has come into wide use in many different forms.
   The legend is that a shepherd built a fire for his night/s vigil, on an outcropping of soft white rock. It burned there throughout the night, and in the morning the shepherd was quick to note that the rock on which he had built the fire, had crumbled under its heat. When the shepherd emptied his water jug into the embers of the fire to extinguish it; a portion spilled into the white powder which turned from a liquid into a mysterious stone-like slab.
   This is what happened. The heat of the fire evaporated, the water which is chemically combined in gypsum rock, thus leaving a crumbled white residue. When the water was dumped into this residue, a re-crystallization took place forming the slab. Essentially, this is the process from the mining of gypsum to the completion of a plaster mold.
   The mineral gypsum is found in large deposits throughout the world in different colors. It is a rock which chemically, is calcium sulfate plus about 20% water. Some of the best known deposits are found in Nova Scotia and in the western part of the U.S. Although usually white, it may be found in shades of pink, yellow, brown or even black. 
     To change gypsum rock into plaster, calcinations of the gypsum rock must be effected. This is done by placing the rock, usually crushed for ease in handling, under heat of 350?F, for a period of time. The same effect may be achieved  at a temperature of 150?F. for a longer period of time. When calcinations takes place, approximately two thirds of the water content of the rock is removed through evaporation.
The first known use of plaster dates back to the building of the first pyramids of Egypt. Then known as alabaster, it was used to decorate and seal the tombs of the Great Pyramids, and was probably used long before the days of the Pharaohs.
      The process of making a plaster mold as we know it today, was discovered about the 18th century. While plaster molds of a different type were known and used for the forming of plastic clay, there were also molds made of wood, sulfur and metal. Lisippos, was one of the first Greek "realists," took plaster casts from faces of living sitters about 300 B.C., sending copies as presents to his friends.
It might be Interesting to take a short trip from a gypsum mine to the mold maker's shop. Large mountains of gypsum rock are blasted out of these deposits, which are shipped to the processing mill where the rock is ground and stored in large silos. From here it is passed through giant rotary calcines which remove about two-thirds of the water content of the rock. Special agents are added to retard the setting action or expansion of the plaster. From the mill it is shipped to the purchaser where it is mixed with water and made into molds.
      Every mold maker has his own methods of making molds. Present methods do not differ greatly from those used a century ago. However, the quality of the plaster and, hence the molds have improved tremendously. The amount of water or plaster used in a mix will Increase or decrease the density of the set. Impurities in the water may retard or accelerate the setting time as well as affect the hardness of the mold. These also affect the life of a mold so great care must be taken.
      Serious trouble can result from using plaster into which particles of set plaster or other contaminating materials have been mixed. Usually the quality of the mold is determined only through casting. However, the outward appearance can be used as a guide. Whiteness, usually denoted by the purity of the plaster, cleanliness of manufacturing, and the finished look of a mold are guides for distinguishing each mold maker's skill and workmanship. 
     A mold made of good plaster can be used for casting a ceramic body, a porcelain body or a beleek body. If a mold is used for casting a colored body. then a white body, the color may show on tile first six to eight castings of the white body. It is therefore, better to use separate molds for casting colored slip. 
      The question may be asked, "Why is a plaster mold used in the casting of ceramics?" When water is mixed with plaster, millions of small crystals interlace and cause the plaster to set. Approximately thirty per cent of the water evaporates when the mold has dried, leaving it porous. This gives the mold room to absorb the water from the slip.

The Use of Molds
    It often happens that a mold is ruined before it is used and likewise a mold can be ruined before it has lost its usefulness. First of all, proper drying is a very important factor in the life of a new mold. When possible, new mo'ids should be dried in the sunshine and where air circulates freely. Molds should never be opened to dryas this may result in warping which causes the seams of the mold to spread and allows the slip to run through. Once properly dried and used with care, the number of casts made from one mold will average one hundred. If sharpness of detail of the cast is a requirement, some molds having' fine details may not give quite as many. Others with a plain surface will cast well over a hundred times before the mold will require replacement.
   It is important that emphasis be made on proper drying of the mold, No plaster mold. should' ever be exposed to temperatures above 125?F. since this will cause the mold to crack and crumble.
     In normal casting, a mold should not be cast more than three times a day. Certain molds can be cast only once a day. When excessive casting is practiced, the mold becomes very wet and after a substantial number of casts have been made from any mold, pitting will generally appear. When this hap
pens there is nothing that can be done to correct it.
     Patching a mold is not recommended. On occasion, however, it may be necessary to replace a chip made in handling or shipping. If it is a small chip, then slip will run into this space when it is cast, but this can be trimmed when removed from the mold. If the chip is so large that it would create a hole when the cast is trimmed, it is recommended that the chip be stuck back on with the use of shellac. First the mold should be dry. Apply a liberal amount of shellac (ready mixed orange shellac is very good) to the broken surfaces of the mold and the chip. Light with a match and allow to burn no longer than ten seconds or the heat will harm the mold. Extinguish the flame and press the chip into place, holding it firm for approximately one minute. Any excess shellac that squeezes out should be cleaned off with a small tool and then wiped with a cloth dipped in alcohol. After drying for a day or two, the piece will stay firm.
  Continuous pouring of slip on the same spot of a mold may cause the spot to crystallize and become non-absorbent. Once a mold has a crystallized spot there is no way to correct it. This spot is caused by the silicate in the slip and indicates that the slip was not thoroughly agitated before pouring. Proper agitation of the slip before casting is very important and is mentioned here because it affects the casting quality of the
   Casting is somewhat an art of it's own. While there are hundreds of basic tips and hint, most are discovered when casting each mold for the first time... For some basic tips, refer to our Free Classroom with photo step by steps on casting. 


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