This batch of 64 stones have just been tumbled in a 4lb barrel with tin oxide “pre-polish” powder. This type of tumble is sometimes seen to be optional, with many stone collectors/polishers simply moving straight to the final “pro-polish” tumble once the stones have been smoothed with a fine silicon carbide grit (e.g., 320 or 400 mesh). However, it has become more common for this stage to be included. [Up-Date 2021: My supplier no longer stocks Pre-Polish tin oxide so I am moving to a one-stage polish approach – see this Post for more information.]
For the “pre-polish” stage, I use a tin oxide powder of 5 microns. A “micron” is an abbreviation for a “micrometre”, or a millionth of a metre, that is, one-thousandth of a millimetre (about .00004 inches). A human hair is, on average, about 75 microns across. Other media can be used for pre-polishing. For example, Steve Hart in his “Modern Rock Tumbling” (2008), reports that he uses 1000 mesh silicon carbide grit which is 4.5 macrons in diameter (page 39). [320 mesh silicon carbide grit is about 29 microns, according to the Washington Mills website.]
The aim of the “pre-polish” stage is to produce a smoother stone than possible with coarser tumbling media. I follow the recommendations that came with the tumbling material I bought from the Rotorua Rock and Gemstone Shop. These are to tumble stones in “pre-polish” powder for between three and five days (note that this is for less than the one week or more recommended for all other stages). “Pre-polishing” assists with the final “pro-polish” stage. After “pre-polish”, the stone is not yet glossy and shiny but is very smooth.
