5.3.

Having secured a kiln or furnace, you will need to fashion a crucible for the work you have undertaken. The crucible must be refractory, lest it succumb to the heat of the furnace. It must be structurally sound, lest it crack and lose the work you have so carefully prepared, and it must be aesthetically pleasing, lest you show yourself to be no true Athanor. The clay you require is generally called low-fire earthenware clay and comes in red or white. If it is to maintain its integrity, the crucible should be all of a single piece of clay, not pressed together from several lumps. It should be free of trapped air bubbles, which might cause it to shatter in the kiln. For this reason, it is imperative that the clay not be rolled out and pressed back together, that it not be flattened and folded on itself.

Figure 5-2. The Crucible

Figure 5-3. The Pedestal

Start with a lump of clay approximately 6 cm x 6 cm x 6 cm and place it onto a piece of paper or plastic to prevent its sticking to the table top. Jam your thumb into the center of the lump and, using your thumb on the inside and your fingers on the outside, open what will be the mouth of the crucible, as shown in Figure 5-2. Rotate the crucible, pressing the walls between the thumb and fingers until the walls and bottom are uniformly 8 mm thick. There should be no seams or cracks, which would weaken the vessel and interfere with the uses to which you will put it.

The crucible must now be shaped into a free-standing cone so that molten material within it will collect in the center. Using the thumbs and fingers, squeeze the base from the outside to form a pedestal, as shown in Figure 5-3. Use your thumb to open up the mouth of the crucible, pressing a dimple into the pedestal at the bottom. Thus the thickness of the walls—even the walls of the pedestal—should be no more than 1 cm thick. The crucible should be 8-10 cm in diameter and 10-12 cm tall.

Figure 5-4. The Lips

Figure 5-5. The Lid

Use a knife to trim the lips of the crucible so that they are parallel to the table top, as shown in Figure 5-4. Manipulate your crucible so that its shape is symmetric and its walls smooth. Then allow it to air-dry for approximately 5 hours. When you return to it, the walls will have stiffened, making it less floppy and easier to work. Make fine adjustments to the shape and surface, eliminating any imperfections. If the surface is too dry, you can moisten your hands or use a damp sponge to make the surface smooth and free of cracks. You can also use a knife, spoon, or tongue depressor to remove bumps or other imperfections. If you do not have time to devote to the work, you can place your crucible into a plastic bag, a veritable time capsule, to prevent its drying out further. When you are satisfied with the shape of your crucible, allow it to air-dry for another 5 hours. After this second drying, the walls will be quite stiff and the surface of the clay cold to the touch. Rub a smooth stone or the back of a spoon over the surface, inside and out, to render the surface glossy. Before allowing your crucible to dry any further, you will need to make a tight-fitting lid for it.

Because clay shrinks as it dries, it is important to make the lid before the crucible has completely dried. Roll out a fresh piece of clay into a pancake, invert your crucible as shown in Figure 5-5, and place it onto the pancake. Scratch your initials into the base of your crucible so that it may be identified. Use a knife to trim excess clay from the lid and then set the crucible on its pedestal. Press the lid slightly into the crucible so that it takes on the shape of a shallow dish, perhaps 1 cm deep. Use your thumb and fingers to make the lid smooth and symmetrical. With the lid in place, allow your crucible to completely dry and scratch your initials into the lid for identification. You may place it into an oven at 130C (270F) for an hour to hasten its drying. Record the dry weight of crucible and lid in your notebook.

Your crucible is now bone dry; you have eliminated any liquid water which was present when the clay was plastic. You will fire it to cone 05 in a kiln, driving off bejeesical water according to Equation 5-1. If this equation is correct then the weight of your crucible should be less after firing than before. In fact, pure, dry kaolinite can be expected to lose 14% of its weight upon firing. A clay body, however, may include non-kaolin materials such as sand or crushed pottery. This "grog" may not lose weight upon firing and so a clay which contains grog may lose less than 14% of its dry weight. The crucible of Figure 5-5, for example, lost only 13% of its dry weight upon firing. Such a weight loss is consistent with a clay which is 93% kaolinite and 7% grog. Record in your notebook the weight of your crucible and lid after firing and compare it to the weight before firing. The time you spend working on your crucible is time well-spent. The lid must fit tightly if you are to successfully make metal in Chapter 9. Its walls must be uniform and strong if you are to successfully make lime in Chapter 10. Its interior must be a conical if you are to successfully make glass in Chapter 13. It takes less time to make one crucible well than to make several of them poorly.

Do not neglect your art, my child. Without a mastery of refractory materials you will be ill-prepared for the work ahead. You must know your clay and be able to fashion it into any form which you require. You must know the fire, to administer more or less according to the properties of the clay. Only then will you be able to keep in the heat and withstand it. Only then will you be worthy of the name, Athanor.

ImportantQuality Assurance
 

When your crucible can be filled with water without leaking or reverting to a plastic state, when its shape is pleasing and its integrity sound, when its lid is tight-fitting, then will you be prepared to continue in the pyrotechnic arts. Include a photograph of your crucible in your notebook and compare its dry weight before firing to that after it returns from the kiln.