Stage Three in tumble polishing involves tumbling stones in 220 grade silicon carbide grit. (The first Post in this series, using 40 Riverton stones, can be found here.) Usually stones tumbled in 220 grit have already been tumbled in 100 grade grit (Stage Two), mainly to “shape” them and start the surface smoothing process. Sometimes a stone found on a beach or river bank may already be well rounded and smooth and it can skip Stage Two. The following outlines what is involved in Stage Three and some of the preparation for Stage Four, including sorting and inspection.
I place the 40 Riverton stones in the 4lb tumbling barrel. Plastic beads, water and 220 grit are then added. I use about 14 tablespoons of beads to bring the load level up to about two-thirds. Each stage of tumbling reduces the size of the stones so progressively more beads are needed unless additional stones (which are of a similar smoothness etc.) are added. As with Stage Two, I add seven tablespoons of grit.
I tumbled these stones for nine days and two hours (seven days is the minimum recommended time). Sometimes I may tumble a Stage Three barrel for longer, up to 11 or 12 days, depending on the state of the stones and whether I am preoccupied with other things.
I empty the slurry from the barrel, wash the stones, clean the barrel and then tumble the stones in about a teaspoonful of grated Sunlight soap for 18 hours (in the same manner as described in my earlier post on Stage Two). This soap wash is very important not only to remove the last of the slurry from the stones but to clean the barrel before the next Stage. I have only one 4lb barrel so I am using it for all Stages. To minimise contamination of one grit by another, thorough washing is required.
When I weigh the washed stones, they come to 1333 grams, having started the Stage at 1470 grams. This means they have lost 137 grams or 9.3% of their mass. This is more than expected, given that they lost less, 8.4%, during Stage Two. Generally speaking, I find that much more is lost during Stage Two (100 grit tumble) compared with Stage Three (220 grit tumble). This batch of stones is unusual in this respect, and I am unsure why. Right at the beginning, before Stage Two, they weighed 1605 grams so the two tumbling stages have entailed the loss of 272 grams which is 16.9% of their weight.
In a previous Post, I described the inspection of each stone after the 100 grit tumble. I do the same inspection after the 220 grit tumble, to check that each stone is ready to go on to Stage Four to be tumbled with 320 grit. In general, the smaller marks and pits on the stones should be worn away but this does not always happen.
For example, Stone 11 had a very minor superficial crack before Stage Three. Afterwards, it has been significantly smoothed out:
You will note some differences in the before and after colours of the stones in this Post, due to the fact that lighting conditions were different when the photos were taken.
Stone 19 has also had a small crack made less pronounced:
These stones would in fact benefit from another tumble in 220 grit for a week to further reduce these “imperfections”.
The other stones with larger cracks and pits emerged from the 220 tumble with them reduced but but still apparent. For example, Stone 1 has a number of small pits that arise because it is a stone of volcanic origin with small gaseous pockets. Wearing away the stone will often simply reveal and expose more such pockets:
Such a stone as Stone 1 will always have these problems in gaining a smooth surface but it can still polish with some success.
Stone 15 has a small gouge in the side that has smoothed somewhat but by no means in a significant way:
A stone like this needs to be re-tumbled in 100 grit.
The surface crack in Stone 20 is still noticeable and requires at least another tumble in 220 grit:
The medium-sized pit in Stone 9 is still apparent, as is the deeper pit in Stone 10:
I would normally seriously consider discarding Stones 9 and 10 and not continuing with them due to these problems. Putting them through the whole polishing process would likely result in the white polish accumulating in the pits and blemishing the finished product. The rest of the two stones would polish quite well but my preference is to try to produce a uniformly polished stone. The only exception would be a stone of significant interest, too valuable for its beauty to discard for a partial blemish. However, for the sake of the demonstration in this series of Posts, I will put all 40 stones through the next stage of tumbling.
I will end this Post with some photos showing the relative sizes of the stones being tumbled. The larger the stone, the more you can tumble it, the more often you can repeat a stage. The smaller the stone, the more likely it will wear away to very small or nothing in the tumbling process. At this stage in the polishing, Stage Three, Stone 1 is approx 7.75 cm long and 4.5 cm wide, Stone 20 is 4.25 cm long and 3.75 cm wide, and Stone 40 is 1.75 cm by 1.5 cm.
I am about to hit the road for four weeks, travelling to the South Island, including to Riverton again where more beach stones will be collected. So there will likely be no more Posts for at least this time.