This important instrument is used alike by the scientific metallurgist, the practical founder, and the amateur. The shape of the crucible and the material of which it is made vary very much, the selection of a suitable article depending upon the nature of the substance to be heated, and particularly of the flux used. Black-lead crucibles are largely used for melting metals, common and precious. Good crucibles of this material withstand sudden changes of temperature, and may be used over and over again, and the smoothness of their surface obviates one great source of loss, as the particles of melted metal do not adhere to the sides.
The metals employed for making crucibles are platinum, gold, silver, and iron. Platinum resists intense heat, but is easily acted upon by caustic alkalies and by the fusible metals, or any compound from which they may be reduced. Gold is too expensive for crucibles, except in important and delicate experiments. Silver crucibles and dishes are used for fusing caustic alkalies. Crucibles of iron are used for roasting many chemical solutions; and they are also used for melting the more fusible metals, such as lead, zinc, tin, etc.
It will rarely pay the amateur to try to make a crucible, as they can be bought so easily and cheaply; but sometimes it may be found necessary to do so. The material will, in general, be some refractory kind of clay, - good fire-clay answering well. Where no very high degree of heat is to be employed, the clay may be mixed with sand; but if the crucible is to be exposed to a very high temperature the mixture of sand and clay will soften, if it does not actually melt. In such cases coarsely powdered fire-brick or old crucibles should be substituted for sand.
The materials, having been ground and kneaded, are generally molded by hand upon a wooden block of the shape of the cavity of the crucible. Another method of shaping a crucible consists in ramming the ingredients into a suitable mold, formed of steel or gun-metal.
A writer in the Journal of the Society of Arts has devised a very neat and expeditious method of forming small crucibles by pouring "slip," that is, clay mixed with sufficient water to give it the consistence of cream, into porous molds, made of plaster of Paris. A series of these molds are placed upon a table and filled with the semifluid composition. By the time the whole (say 50 or 60) are filled, the "slip" may be poured out of the one first filled, leaving only a very small quantity behind to give the requisite thickness to the bottom. The second and third may then be treated in the same way, until the whole number has been attended to. In each mold a perfect crucible is formed by the abstraction of the water of that portion of the "slip" in immediate contact with the plaster; and the crucible is either thicker or thinner in proportion to the time this absorbent action has been allowed to go on. Seventy or eighty crucibles may thus be easily made in less than fifteen minutes. The molds and their contents are next placed in a stove or slow oven. In a short time, from the contraction of the clay in drying, the crucibles may be removed; and the molds, as soon as they have become dry, may be again filled. By care they will last for years.
The amateur chemist will often find that the bowl of a tobacco-pipe will make a very good crucible. The hole at the bottom should be well plugged with a little of the ground tobacco-pipe made into a paste with pipeclay and water.