There are seven stages in the tumble polishing of stones. The first stage is acquiring the rough stones. Stage Two involves tumbling rough stones in a solution of water and 100 grade silicon carbide abrasive grit. Stage Three entails tumbling the now smooth stones in 220 grade grit. The 40 Riverton stones being tumbled for this series of Posts were next placed in 320 grade grit, Stage Four, on 10 March after I had returned from my South Island stone collecting trip. This Post is the sixth in this series, the first Post being here.
Note that in this series I have counted the collection of stones as Stage One. Nearly all other accounts of tumble polishing call the first actual tumble Stage One whereas I have called it Stage Two – and this affects the numbering of all subsequent Stages.
Furthermore, while I initially put nearly all the stones I collected through Stages Two, Three and Four, I no longer do this as a matter of course. More often these days, I consider starting a stone at Stage Four, some of them I start at Stage Three, and I rarely start a stone at Stage Two. This is because I mainly collect smooth beach stones that do not need shaping and/or smoothing as much as stones found in rivers or on dry land.
So, for Stage Four, I place the 40 Riverton stones in the 4lb tumbling barrel. Water, plastic beads, and 320 grit are then added. I use about 16 tablespoons of beads to bring the load level up to about two-thirds, as the stones have gotten smaller with each tumble. I don’t usually put just the same load of stones through each stage – I usually do two or more loads at the lower grit grade and then sort each load in relation to which Stage they should go to next. In this way I am able to collect enough stones for a good-sized load for each stage.
I use a number of compartmented plastic trays, 45 cms long, 30 cms wide and 8 cms deep, for sorting and storing stones in order to collect enough for a tumble load.
Back to the Riverton stones being tumbled for this series on the Stages of tumbling:
As with Stages Two and Three, I add water and seven tablespoons of grit (320 grade) to the barrel for Stage Four. I then tumble these stones for nearly 10 days. The recommended time for Stages Two, Three and Four is seven days each but this is a minimum and I often leave the tumbler going for another three or four days per stage.
So, after nearly 11 days tumbling, I empty the tumbler into a stainless steel strainer (sieve) labelled for 320 – it is used only for stones straight after being tumbled in 320 grit (to prevent contamination with other grades of grit).
The strainer is placed over a bucket and I run water over the beads and stones to wash the slurry away. I then clean the empty barrel, using a toothbrush labelled for “320” use, also using a paper towel (as previously described in detail for Stage Two).
After thorough washing, I put the stones back in the barrel. I add water along with a few flakes of sunlight soap and place the barrel back on the tumbler. In this case, for these 40 stones, I tumble them in soap for just over three days. Sometimes I run a soap tumble for only a day, but as I am going to re-use the 4lb barrel for the next stage, pre-polishing, I want it to be as clean as possible to avoid contamination with previous grit.
So on Friday morning, 23 March, I empty the soap-tumbled stones out of the barrel and wash them and the barrel.
Once the stones are dry I put them on the scales and discover they weigh 1261 grams.
At the start, before any tumbling, the stones weighed 1605 grams, losing 8.4% of that weight during the 100 tumble (ending up weighing 1470 grams after the 100 stage). During the 220 grit tumble, 9.3% of this 1470 grams was lost (the stones ending up weighing 1353 grams). Stage Four, the 320 grit tumble, resulted in a reduction of a further 92 grams, which is 6.8% of 1353 grams. In total, through Stages Two, Three and Four, the stones have lost 344 grams of their original weight of 1605 grams, which is 21.4%, just over one-fifth.
The amount of material lost during tumbling is dependent on a number of factors – the length of tumbling, the softness of the stones, any chips lost from the stones, and so on.
The next Post is about the state of the stones after the 320 grit tumble.