The oxidation reaction of magnesium (Mg) with oxygen (O) in a ceramic crucible is commonly performed in high school senior science chemistry to produce magnesium oxide (MgO). This leaves behind a black carbon-like residue, which becomes strongly bonded to the crucible surface.
The link to this website has more detailed information:
Armstrong, Jacob A. 2005, ‘Combustion induced reaction of magnesium with a silicate coated crucible surface’, Journal of Young Investigators https://www.jyi.org/2005-february/2005/2/9/combustion-induced-reaction-o...
Trying to clean glassware and ceramic crucibles that have chemical/metal oxide residue stains is problematic and can be a time-consuming exercise. To be honest, I don't think there's a way to get ALL the staining off of the crucible. When the magnesium burns, it often burns off the surface of the ceramic. Once that is not there, the stains are very difficult to remove.
A stain in the crucible will not injure it for future use. Science ASSIST recommends re-using the crucibles for this activity, rather than spending valuable time trying to remove the staining left from the burnt residue. Crucibles, like glassware, are consumable items and are cheap enough to replace by purchasing new ones from science suppliers.
Safe work practices: Any chipped, cracked or broken crucibles should be disposed of safely. Science ASSIST has developed an information sheet for the safe disposal of laboratory glassware see AIS: Lab glass and porcelain disposal. Safe and environmentally appropriate ways of cleaning and disposal of the waste from the chemicals used in the cleaning process should be non-hazardous and not cause environmental harm. A site-specific Risk Assessment should always be carried out and used in conjunction with Safety Data Sheets.
Cleaning: Scrape out the contents of the crucibles using a wire brush to remove the more stubborn bits and, if necessary in order to make it clean, scour it thoroughly with moist powdered pumice or fine sand. (Gumption is also a good abrasive cleaner). Wash them thoroughly in hot soapy water and then make sure that they are dry (this removes any residual magnesium or magnesium oxide) before soaking in either:
- dilute solutions, followed by concentrated hydrochloric acid
- dilute solutions, followed by concentrated nitric acid
Soaking times vary from 5 to 10 minutes for general glassware or up to 12 hours for problem stains. After soaking, rinse 3 times in tap water and then once in distilled water and air dry. After that anything that is ingrained into the ceramic glaze is not going to come off.
Science ASSIST strongly advises against preparing or using the following solutions for the cleaning of glassware or porcelain because they are extremely corrosive and/or carcinogenic and/or potentially explosive.
- Aqua regia
- Piranha Solution
- Hydrofluoric acid
- Chromic acid (also known as dichromic acid)
- Fuming sulphuric acid
See also a related question: Dichromic acid glass cleaner waste
Dungey, Barbara, 2006, The Laboratory: a science reference and preparation manual for schools. Revised edition. Contemporary Press Pty. Ltd., Victoria
Chemwatch Gold. 2014. Safety Data sheet: Magnesium.
Chemwatch Gold. 2014. Safety Data sheet: Hydrochloric acid.
Chemwatch Gold. 2014. Safety Data sheet: Nitric acid
https://jr.chemwatch.net/chemwatch.web (subscription required accessed 2015).