There are seven stages in the tumble polishing of stones. The first stage is acquiring the rough stones, the topic of The Seven Stages in Tumble Polishing Stones: Stage One, Stone Collection, Riverton, 2-6 November 2017. The stones to be tumbled as illustrations in this series are described in The Seven Stages in Tumble Polishing Stones: The Selected 40 Stones. Stage Two involves tumbling rough stones in a solution of water and a low grade abrasive grit, in this case, 100 grade silicon carbide. (Note that if a beach stone is quite smooth, this stage, and even the next one, could be skipped. If a stone is quite rough or jagged or has pits in it, it might need to go through this stage twice.) A 4lb tumbling barrel will be used as the largest stones are too big for the smaller 3lb barrel.
Note that in this series I have counted the collection of stones as Stage One. Nearly all other accounts of tumble polishing call the 100 grit tumble (or the tumble with some other coarse grade grit) Stage One whereas I have called it Stage Two – and this affects the numbering of all subsequent Stages.
NOTE: March 2021. My supplier now stocks 80/100 grit instead of simply 100. The former contains a range of grit sizes between 80 and 100 which actually has little different effect on stones, being only slightly more corrosive.
First, the stones are placed in the 4lb rubber barrel and plastic beads (also known as pellets) are added. The beads help to “carry” the grit to the surfaces of the stones, making the process more efficient and thorough. When not enough stones are available to make a good-sized load, beads are also used to bring the contents of the barrel up to two-thirds full, the ideal level of good tumbling (see this video for a demonstration of why two-thirds is a good level). These beads were bought from my stone tumbling supplier in Rotorua. Once beads are used with a particular grade of grit, they are stored in a marked container and used only with the same grit – this prevents contamination of one kind of grit by another (each grade of grit produces a consistent set of minute scratches on the stones, contributing to the final polished product). I have a kitchen dessert spoon that I use for 100, 220 and 320 grit beads, and another is used for the pre-polish and pro-polish beads, again to minimise cross-contamination. I added 10 dessert spoons of beads to the barrel, then added water to just above the level of the stones and beads.
Silicon carbide grit is added next. I use seven tablespoons of 100 grade grit for a 4lb barrel load (the guidelines are “6 to 8 tablespoons”). I keep different grades of grit in their own containers, and each container has its own tablespoon measuring spoon (to avoid cross-contamination):
The lid is then placed on the barrel and the barrel placed on the tumbler. It will now rotate continuously on the tumbler for at least seven days (the recommended time). Longer is fine – I often tumble for about two or three days more. The grit breaks down after a while, but then it can act like a finer grit.
Finally, I fill out the Tumble Log, noting where the stones are from, the type of barrel and grit used, how many beads or pellets were put in, the time and day the tumbler was started, how much grit was used, and the weight of the stones. This is my 189th barrel since I placed the first one on the tumbler in March 2016.
Nine days later, on Friday 24 November, I turn the tumbler off and prepare to open the barrel to see how the stones have fared. I grab a yellow plastic colander that hangs on the wall under a calendar and then select the appropriate stainless strainer from nearby, one labelled for 100 grit. There is a strainer dedicated to each grade of grit and polish, to avoid cross-contamination. I place a bucket in the sink then put the colander and strainer across the top. I will pour the stones and beads into the strainer, to wash them, and the colander will act as a safety net in case some spill over the side of the strainer. I also have some paper towels handy for cleaning and mopping up purposes.
I carry the 4lb barrel over to the sink and take the top off. Inside what is first apparent is the grey liquid slurry that contains the material that the grit has worn away from the stones. Some broken down grit will also be part of this mix. I hold the lid under the stream of water from the tap so that the slurry and beads on the lid are washed into the strainer. I then use a paper towel to wipe the lid, especially the outer rim which is the connection to the barrel’s rim – when the lid is placed back on the barrel, this connection needs to be clean so that the seal is a good one. No matter how much water is run on the lid to clean it, a paper towel will always pick up more.
The contents of the barrel are then poured carefully into the strainer, usually with water running to assist the clearing and cleaning process. The beads and stone are covered in the grey slurry, and at the bottom of the barrel will be a muddy sticky layer where the slurry has started to settle in the short time it has stopped tumbling. Some of the smaller stones are likely to get stuck in this and extra water will be needed to free them. We reach the point where the stones are relatively clean and 99% of the grey slurry has been washed through into the bucket beneath.
The next steps are to empty the slurry, clean the barrel, and do a final rinse of the stones before putting them back into the barrel for a soapy tumble.
The slurry in the red bucket in the sink is poured into a bucket on the floor. There, over the next few days, the slurry will settle. The water on top can eventually be poured off and the accumulated sediment in the bucket will be buried or otherwise disposed of. Slurry should never be poured down the drain in a house as the sediment will accumulate in the S-bend, turn as hard as concrete, and block the pipe. Once the slurry has been poured out of the red bucket, it will need rinsing. The sink in my shed has no S-bend, drains through a hose out onto the ground a few metres away, so sending some slurry down it is ok.
The 4lb tumbler barrel needs to be well washed out. Then attention needs to be given to the rim of the barrel, where the lid “seats” and seals. Stubborn slurry needs to be removed. I use a toothbrush for this, and have a brush for each grade of grit and polish (with the grade written on each). Finally I wipe the rim down with a paper towel.
The stones and beads in the strainer are then given a thorough rinse, to remove as much slurry from them as possible. After that, I place the stones then the beads back in the 4lb barrel, topping them up with some more beads (as the size of the stones has been reduced due to the previous nine days of tumbling and it is a good idea to refill the barrel to two-thirds). Water is added to the level of the top of the stones, and a little soap is sprinkled on top. I have seen various sources recommend the use of a mild low-sudsing soap, with Sunlight mentioned. I have managed to track down an outlet for Sunlight soap and I grate the cakes. The lid is then placed on the barrel and it is put on the tumbler for a few hours. This cleans the last of the slurry off the stones, and also cleans the inside of the barrel.
The two last jobs for this phase is to clean the bucket (it contains the remnants from the final rinse of the stones) and to add details to the Tumbler Log about the time the tumbling ended, how long the tumble period was, and to note when the soap tumble started.
After 12 hours, I take the barrel off the tumbler to remove the stones and beads from the soapy water. They are placed in the strainer again and rinsed. The stones are placed on a paper towel. I remove as much water from the beads as I can then they are spooned into the “100 grit” storage container.
When I weighed the dry stones, I found they were now 1470 grams, having lost 135 grams during the tumbling process. This means they lost 8.4% of their weight. In my experience, this is towards the lower end of the expected range, meaning that perhaps at least some of the 40 stones being tumbled are relatively hard. I add the end weight to the Tumbling Log.
The last thing to be done before the stones go to the next tumbling stage is to “sort” them. This means that I look at each stone in good light while it is dry. I look for lack of smoothness and consider whether a stone would benefit from more tumbling in 100 grit. A relatively small imperfection in smoothness is ok as the 220 grit tumble will remove that. In rare cases I will decide that a stone can skip the 220 grit stage and move straight to the 320 grit tumble. Sometimes, especially after the finer grit tumbling, I use a loupe (magnifying glass) to check for cracks, pits and imperfections before deciding what to do with a stone. This loupe can magnify three times (the magnification I used most often) or six times or nine times.
The next Post considers the inspection and sorting of the stones prior to them being tumbled in 220 grade silicon carbide grit.
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