A crucible cover is an extremely heat-resistant lid used to contain the contents of a crucible. It is traditionally made of clay, though other materials, such as strong metals, may be used. Unlike a typical lid, a crucible cover is wider than the mouth of the crucible. This helps keep the covering loose enough so that some gas can escape while the contents are being heated.
Many crucible covers have a handle for easy management. It can be a loop at the top of the cover or a flat handle that extends from the edge of the piece. Some covers have a lip all around the edge which can be grabbed at any place.
It is important that a crucible cover be able to handle high temperatures, because a crucible is used to melt materials such as glass and metal. The cover may also be used to contain chemicals while they are being heated in a laboratory setting. Materials that are too weak could cause the lid to explode, crack, or break. Sometimes the heat can also deform an inadequate cover. Most cover manufacturers will indicate the top temperature a crucible cover can handle.
Depending on the task, a crucible cover can be used for containment, protection, or both. In some cases, it can prevent the contents of the crucible from escaping, though not blocking all emissions. It may also help keep elements from the atmosphere, such as oxygen, from entering the vessel.
Crucible covers can be purchased with a crucible or individually. Due to the intensity of heat they handle, crucibles can only endure a certain amount of wear and tear before they start to crack or otherwise become degraded. Sometimes a cover will outlast its vessel, though the opposite can happen as well.
When crucible covers are made of ceramics, the seal between the lid and the vessel may not always be secure. This is due to differences in the thickness of the crucible walls which usually cannot be entirely avoided. For this reason, a metal crucible and cover may be more reliable for tasks where a closer seal is required.
A crucible can be used with or without a lid, depending on what is being heated. In a laboratory setting, it is usually used for both safety reasons and to control what enters the vessel. A lid is often not used with larger crucibles that are placed into furnaces for melting metal.