Crucibles are usually made out of porcelain and they may be heated to very high temperatures, either by a direct Bunsen burner flame or in an oven, without breaking. They are delicate in other ways, however, and crack or break quite easily, espcially when set down on cool counter tops while still hot.
You should always check a crucible over very carefully before you use it, looking for cracks or other defects that may have developed over the lifetime of the crucible. Show the lab instructor the crucible if you see anything that might look questionable about its suitability for use.
The crucible cover is usually tilted slightly so as to let hot gases escape more easily. The crucible should be heated slowly with the Bunsen burner held at a distance at first and then brought in closer as the crucible gets warmer. The crucible can be heated until it is red hot. Notice how the Bunsen burner is centered underneath the crucible and the flame is turned up high and comes in contact with the bottom of the crucible. Always place the hot crucible on a wire gauze, NEVER directly on the desk top. The cold surface may crack the crucible, thus ruining your experiment, and/or dirt, paint, etc., are easily fused onto the hot porcelain, thereby changing its mass. Crucibles are usually very hot when you first put them on the wire and it important to let them cool down to room temperature before putting them on a balance. You can check to see if it is safe to weigh them by bringing the palm of your hand down slowly over the crucible. You will be able to feel the heat rise at first. The crucible will be cool enough for the balance when you can let your palm barely touch the lid and it just feels slightly warm. Crucibles are used for chemical analysis of some kind and their masses are usually determined on three or four-place balances. It is imperative that they be carried by use of tongs (with a wire held underneath each to help keep them steady) because oil from fingertips will add to the mass of the crucible.
Send questions, comments or suggestions to
Gwen Sibert, at the
Roanoke Valley Governor's School
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