This is the seventh Post in this series – the first one can be found here. This current Post concerns a process that occurs after Stage Four and prior to Stage Five.
After the stones have gone through Stage Four, being tumbled with 320 grit, they are a lot smoother than previously. Normally I would inspect each stone for any “imperfections” before putting them through the final two polishing stages. This involves looking at each dry stone under a bright light, sometimes with the aid of a magnifying glass. Those stones that fail this inspection would be tumbled again in 220 or 320 grit to remove the “imperfections”. However, for the sake of this series of posts, I am going to put all 40 stones straight through to the polishing stages. The following ten stones are ones that I would normally consider holding back at this stage.
Stone 31, one of the smaller stones, is in the worst condition, with a number of gouges, pits and indentations. The stone is probably made of brittle material which predisposes it to lose parts even during the tumbling process. Usually I would discard this stone altogether.
Stone 11 initially had a very minor superficial crack which had been smoothed out more and more during the tumbling process. However, after the 320 grit tumble, there is still about 1 mm of the crack remaining. I would usually give this stone another tumble in 320 grit before going on to the next Stage.
Stone 19 still has a small section of crack apparent, and another small crack has appeared as a result of the 320 grit tumble. These are likely to be at least partly due to the patterning on the stone, of the white streaks throughout the grey rock. Further tumbling in grit is likely to smooth out one or both cracks but create others. It is worth putting this stone through to the next Stage to see how it emerges from the polishing process, mainly because of its interesting pattern.
I noted after its tumble in 220 grit that Stone 1 has a number of small pits that arise because it is a stone of volcanic origin with small gaseous pockets. Tumbling the stone reveals and exposes more such pockets. This is apparent also after the 320 grit tumble. It is always hard to judge whether further tumbling in 320 grit will improve its smoothness or simply reveal more gaseous pockets.
The small gouge in the side of Stone 15 has been further reduced by the 320 grit tumble, but remains larger than I would normally like at this stage.
The surface crack on Stone 20 has been significantly reduced by the 320 grit tumble but requires this to be repeated as it still catches a fingernail crossing over it.
Stone 9 has a pit in its side, which has been smoothed out further by the 320 grit tumble. This indentation could be made up of softer material that the surrounding rock so it may continue to erode faster if re-tumbled in grit.
Stone 10 still has its deep pit, and a couple more small pits have also appeared. This stone could be discarded.
The indentation in Stone 4 has smoothed out further and it is possible that this might not affect its final polish.
Finally, small cracks in Stone 25 still remain but are very small.
The next step is to tumble all 40 stones in Pre-Polish mixture – see the eighth Post in this series.
3 thoughts on “The Seven Stages in Tumble Polishing Stones: The Stones After 320 Grit and Their “Inspection” and “Sorting” Before Stage Five”