The Seven Stages in Tumble Polishing Stones: Stage Six, Pro-Polish Tumble, 1 October to 15 October 2018

NOTE: March 2021. My supplier now stocks only one grade of tin oxide polish powder instead of two. This means that Stages Five and Six can be replaced with one stage. Details can be found in UP-DATE OF “The Seven Stages in Tumble Polishing Stones” – One Less Stage. I will leave the original Posts unchanged as the detailed activities of either Stage can be applied directly to the use of one polish stage.

(This is the tenth Post in this series – the first Post can be found here. Apologies for the delay that’s interrupted this series.)

In his book, “Modern Rock Tumbling” (2013), Steve Hart, a mechanical engineer, comments that how a stone becomes shiny after polishing is both a miracle and a mystery:

There is no one who really knows what happens inside the barrel when polish is added to the rocks. The Silicon Carbide stages were relatively easy to understand, because the process was primarily mechanical erosion or abrasion. Not so with polish. There are “mysteries” that don’t add up… There are theories that a given polish may liquefy the top layers of molecules on the rocks and rearrange them so the surface is smoother. Others say it may be an acid/base pH thing, or perhaps a frictional heat buildup phenomenon which causes the rock surface to flow… Another theory is that the rock surface is simply mechanically hammered… down to a smoother condition… After really trying to understand the polish process, I had to give up, because no one… really knows what’s going on (pages 40-41).

Stage Five of tumble polishing used a “pre-polish” tin oxide powder of five micron size. Stage Six uses a “pro-polish” tin oxide powder of one micron size. In general terms, I am following the instructions provided by Colin Simmons of the Rotorua Rock and Gemstone Shop from whom I bought my tumblers. He also sells the two grades of tin oxide powder. Colin recommends that the Pro-Polish tumble should be for at least seven days, and he states that the pro-polish mixture is retainable and reusable for about six batches of tumbling as long as it does not become contaminated. In this instance, I used a pro-polish mix that I had used five times before.

The procedure for starting the Pro-Polish tumble is the same as for the Pre-Polish stage, using the key ingredients: the 4lb tumbler, the white pro-polish tin oxide powder, a plastic shaker with a tight lid, and the plastic beads kept for pro-polish tumbling (see the relevant previous Post for details):

I try to tumble a pro-polish batch for at least 10 days, even though Colin Simmons states that it is okay to tumble for only seven days (as the minimum). In this case, I had taken on a temporary job which kept me busy while the stones were tumbling and I left them for a total of 14 days and five hours before taking the barrel off the machine. The process of taking the stones out of the barrel and washing them is the same as detailed for the Pre-Polish Stage (Note, January 2022 – I have just come across an observation from an experienced polisher that using a metal colander to rinse stones at the polish stage tends to put metallic streaks on the stones. I have not noticed the same issue with a metal sieve):

I then use my pro-polish toothbrush to finish cleaning the barrel before the next stage:  

However, instead of then putting the stones in a soap tumble, they undergo a longer tumble wash in borax as a burnishing process, which is Stage Seven, described in the next Post in this series.