Tony Withers: Lab: Cleaning Crucibles

Instructions for cleaning Pt crucibles

To remove contaminants such as Fe and Mn from platinum labware, we bring the platinum into contact with a silicate or borate liquid. The aim is that at high temperatures the contaminants will partition into the liquid, which can then be quenched to form glass and removed from the crucible. Repeated fusions may be required to reduce contamination to an acceptable level. Liquid compositions with low melting points that can be used for this include lithium metaborate and sodium metasilicate. Transition metal contaminants will tend to color the glass, so a clear glass is usually a good indicator of a clean crucible.

The standard operating procedure for the cleaning process was provided by Lora Armstrong:

• If the crucible was only used to dry a powder & appears clean under microscope after initial wiping out with a solvent, no further cleaning needed.


• If the crucible was used to fuse a glass, or appears dirty under microscope after initial wiping out with a solvent, or if you are suspicious of its cleanliness for any other reason, then it needs to be cleaned further as described below.
  
  

  **Safety note: Avoid skin & eye contact with sodium metasilicate, particularly when it is dissolved in water. It can cause skin irritation and burns. If skin/eye contact occurs, rinse affected area with water. Wear gloves when handling wet crucibles full of sodium metasilicate glass, or when dealing with sodium metasilicate-bearing aqueous solutions. See MSDS for more information. **

  
  
• Make sure crucible is dry. Fill 1/2 - 2/3 full with sodium metasilicate (Na2SiO3; melting temp = 1088°C)


• Put in box furnace at 1100-1125°C and leave at the minimum until the silicate melts, usually 15-20 minutes depending on the temperature.


• Have empty metal coffee can or other metal container next to the oven on a ceramic block.


• Open furnace & use tongs to lift out crucible, quickly pouring as much of the melt as possible into the coffee can & shutting the furnace door.


• Let crucible cool. You will hear & see the glass fracturing as it cools. (Cooling can be sped up by carrying crucible to the sink with tongs & cooling with water but beaware of safety issues above with regards to water. If you cool it this way, wipe off crucible exterior & interior before trying to get out glass).


• Once crucible is cool enough to touch, remove glass. You can gently squeeze the crucible walls in different directions to help with this. Depending on the state of the crucible, glass may pop out easily or may need to be flaked off in pieces with the help of a metal tool. If there is a piece you can’t get out, don’t worry- it can be dissolved in water.
  
  [Note: If more cleaning is needed, the sodium metasilicate melting can be repeated several times before a final cleaning with water ± HCl.]


• Once all or most of the glass has been removed, take crucible to 284 Kolthoff & place in beaker filled with water to dissolve any remaining glass. Empty & replace the water a few times. You can use a tool to try to remove any stuck on glass if needed (be aware of safety issues described above with regards to water).


• Dry off the clean crucible once you’re satisfied there is no more glass left.


• Optional: If desired, crucible can be cleaned further by putting in fume hood & filling with a mixture of water and dilute HCl for however long you want.
  
  

  **Box furnace safety: Use lab coat, protective eyewear/welding goggles/face shield, and heat resistant mittens. You should also know where the emergency first aid kit is kept in each lab, and where to find the fire extinguishers.**

  
  
  

LSA 2012.

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