Plaster and lime are manufactured by heating the bejeezus out of gypsum and limestone, respectively. Because the bejeezus is different in each case, however, water for gypsum and carbon dioxide for limestone, they require different temperatures for calcination. Conversion of gypsum to plaster requires a temperature between 128°C (262°F) and 163°C (325°F). If the temperature is too low, the conversion to plaster is incomplete. If it is too high the plaster is converted to anhydrite, which slakes only very slowly. A good compromise is to set an ordinary oven to between 130 and 140°C (266 and 284°F).

Weigh your empty crucible and record the weight in your notebook. Fill it two-thirds full of powdered gypsum, available wherever lawn and garden products are sold, and record the weight of your filled crucible in your notebook. By subtracting the empty weight of your crucible you can determine the weight of gypsum used, wgypsum. Test the pH of your gypsum with wet strip of pH test paper and record the result in your notebook. Place your crucible in the oven for a couple of hours, even overnight if you have access to a laboratory oven which stays on all the time. Remove it from the oven and weigh it again; you should find that it has lost weight, the weight of the bejeesical water that was driven off by the heat. Record the weight in your notebook and subtract the empty weight of the crucible to get the weight of your plaster, wplaster. Divide the weight of your plaster, wplaster by the weight of the gypsum, wgypsum. If your gypsum were 100% CaSO4·2 H2O and if it were completely converted to CaSO4·˝ H2O then the ratio of plaster to gypsum would be 84%. Your ratio may be a little more or less depending on the purity of your gypsum and the completeness of your conversion. Agricultural "gypsum" often contains anhydrite in addition to gypsum. Would this make the ratio of plaster to gypsum higher or lower than 84%? Test the pH of your plaster with a wet strip of pH test paper and record the result in your notebook.

Now that you have some plaster you should make something useful from it. In Chapter 13 you will make glass and we can use our plaster to make a mold for that project. Plaster alone is often used to make ornamental objects or molds for ornamental objects, but by itself it cannot withstand the high temperatures needed for making glass. Glass casters mix plaster with silica to produce an investment suitable for high-temperature molds. We will use one part plaster to one part silica, available in powdered form wherever pottery supplies are sold. To begin with we need to coat the inside of the crucible with investment to keep the model from touching the sides of the crucible. Transfer your plaster from the crucible into a plastic tub for storage and label it. Before you go any further you should read the instructions for weighing by difference . You may think that you know how to weigh things, but a few minutes of extra reading will save you a lot of trouble.

Your crucible should be wet. Fill it with water while you mix your investment. Weigh 20 g of plaster and 20 g of silica into a plastic bag and seal it. You now have a plastic bag containing equal parts of plaster and silica. Turn it end for end, massaging any lumps and mixing your investment as completely as possible. Weigh out 20 g of water into a plastic cup and add a spoonful of investment to it. When the dry powder sinks to the bottom add another spoonful. Continue adding investment to the water, waiting until each spoonful sinks before adding the next. The last spoonful will not sink below the surface and this is a sign that you have the correct ratio of water to investment. Use a spoon or spatula to completely mix your investment.

Figure 10-2. Coating the Crucible, Making the Model

Quickly empty the water from your crucible and pour your investment into it. Tilt the crucible from side to side to completely coat the walls with investment, as shown in Figure 10-2(L). While this investments sets, make a model from clay. Keep it simple. The model shown in Figure 10-2(R) is of a scarab, a noble creature even if it does have only six legs. Make your model approximately 5 cm x 3 cm x 5 cm high. Mix up another cup of investment, this time using 50 g each of plaster, silica, and water. Pour this second batch of investment into your crucible, use a spoon or spatula to pour investment into the features of your model and then plunge it into the crucible, leaving the base sticking up above the investment as shown in Figure 10-3(L). Allow your investment to set for a full hour. Then grab the base of your clay model and wiggle it from side to side until you are able to pull it free from the mold. Use a knife or toothpick or remove any excess clay from your mold. The finished mold is shown in Figure 10-3(R).

Figure 10-3. Making the Mold

Your mold will need to be completely dry before you can use it from making glass in Chapter 13. You can let it air-dry or you can put it in an oven at 130°C (266°F) for a couple of hours. While you are waiting for your mold to dry, let's make some lime. Weigh your crucible lid and record its empty weight in your notebook. Fill your lid with crushed limestone, weigh it, and subtract the empty weight. Record the weight of the limestone, wlimestone, in your notebook. Test the pH of limestone with a wet strip of pH test paper and record the result in your notebook. Fire the lid to cone 05 to convert the limestone to lime. Weigh it when it comes back from the kiln, subtract the empty weight of the lid, and record the weight of your lime, wlime, in your notebook. Calculate the ratio, wlime/wlimestone. If your limestone were 100% CaCO3 and if it were converted completely to CaO, the ratio would be 56%. Because agricultural limestone is not 100% calcium carbonate and because the conversion may not be complete, your ratio may be slightly different from the theoretical value.

Whatever the weight of your lime, weigh half that much water into a glass jar and spoon your lime into it. If the lime is fresh from the kiln it may get hot as it slakes. Test the pH of the slaked lime with a strip of pH test paper and record the result in your notebook. When it has cooled, screw on the cap, label the jar, and save your slaked lime for making paper in Chapter 14.

I think that Unktomi-Hermes must have been talking about the magic of carbon dioxide, which the Sun turns into trees, which the trees return to the air by moonlight, which turns into stone in limestone caverns, and which escapes from that stone in the heat of the kiln:


Its father is the Sun, its mother is the Moon. The wind has carried it in his belly. Its nourishment is the Earth. It is the father of every completed thing in the Whole World. Its strength is intact if it is turned toward the Earth. Separate the Earth by Fire, the fine from the gross, gently and with great skill.

 The Emerald Tablet of Hermes Trismegistos

ImportantQuality Assurance

Compare your ratios, wplaster/wgypsum and wlime/wlimestone to their theoretical values. Compare the pH of plaster to that of lime and explain why they are different. Include a picture of your mold in your notebook.