The cleaning methods for carbon-carbon crucibles mainly include physical and chemical methods.
Physical method
Scrub or fine sandpaper: For cases where the surface burns are not serious, you can use fine sandpaper or scrub to gently polish the burnt black parts. This method is suitable for cases where the surface burns are not serious, but you need to be careful to avoid damaging the crucible surface. Fine sand and water: Use non-angular fine sand (such as 100 mesh fine sand) and water to gently rub the crucible surface to remove stains and restore the surface gloss. Be careful to avoid using too rough sandpaper or tools to avoid scratching the crucible surface. Chemical method
Vinegar and water mixture: Mix vinegar and water in a ratio of 1:2, pour into the crucible and heat to boiling, let it boil slightly for a few minutes, turn off the fire, and wipe with a sponge or soft cloth after cooling. This method is suitable for removing burnt carbides.
Soda water: Add an appropriate amount of water to the crucible, then add two to three tablespoons of baking soda, boil it and let it simmer for a while, then wipe it with a soft cloth after cooling. Soda water can effectively remove carbides.
Citric acid or lemon juice: Use citric acid or fresh lemon juice instead of vinegar, the effect is similar. Citric acid is particularly effective for removing burnt black.
Hydrochloric acid cleaning: For carbon-containing residues or other substances that are difficult to clean with ordinary methods, hydrochloric acid can be used for cleaning. Hydrochloric acid can dissolve most inorganic combustion residues. Note that the hydrochloric acid used should not contain nitric acid, nitrates, halogens and other oxidants.
Nitric acid cleaning: For carbon-containing residues, nitric acid can be used for cleaning. Nitric acid is highly oxidizing and can be heated appropriately during cleaning, but the temperature must be controlled to avoid the crucible from cracking.
Dilute acid boiling: Place the crucible on a heating device and slowly heat it to a boiling state for a period of time until the residue is dissolved or loosened.
Molten salt cleaning: If dilute nitric acid still cannot clean it thoroughly, you can consider using molten salts such as potassium pyrosulfate, sodium carbonate or borax for cleaning. Pour an appropriate amount of molten salt into the crucible and heat it to a molten state so that the molten salt reacts with the residue and dissolves. After the molten salt cools down, rinse the crucible with water.