In general, the attachment on graphite crucible is the residue of inorganic matter after combustion. Most of these residues are soluble. We can use hydrochloric acid to clean up. Some residues containing carbon cannot be dissolved with hydrochloric acid. Here, nitric acid can be used for cleaning, because nitric acid has strong oxidation. Nitric acid can also be properly heated during cleaning.

Graphite crucible needs to be kept clean and bright inside and outside. If there are stains on the surface of the graphite crucible, the fine sandpaper of 100mesh should be wetted first, and then the surface of the graphite crucible should be gently rubbed to restore its luster.   

For substances that need to be cleaned, we can boil them in dilute hydrochloric acid or dilute nitric acid. Note that hydrochloric acid shall not contain oxidants such as nitric acid, nitrate, halogen and so on. If dilute nitric acid cannot wash the stain, it needs to be washed with potassium pyrosulfate, sodium carbonate or borax after melting.

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