Category: Stones Undergoing Polishing

(This is the ninth Post in this series – the first Post can be found here.)

Two of the stones have suffered some minor damage in the Pre-Polish tumble, probably from bumping into other stones. This is despite careful handling and the use of plastic beads to cushion collisions in the barrel. This will happen from time to time due to the brittle make-up of some stones. Stone 6 has lost a small chip in its side:

Stone 12 has has lost three or four small pieces, the largest being apparent in the photo below: 

In such cases, stones are usually returned to Stage Two or Three, to be re-tumbled with 220 or 320 grit to get rid of the pits. However, I will put them through to the Pro-Polish stage for the sake of this series of Posts.

The following are the stones as they looked upon initial collection from the beach at Riverton and then upon completion of Stage Five, Pre-Polish. In both instances, the stones are dry. Note that the same sized segment of graph paper is provided in these comparative photos. At present the stones look a lot smoother and shinier, and are slightly smaller. Their colours are clearer and brighter, with any patterns being more obvious.

Stones 1 to 5:

Stones 6 to 10:

The grain of Stone 6 is now apparent. The wearing away of Stone 7 has led to the band across its lower part to be uncovered more and thus made to appear larger.

Stones 11 to 15:

Stone 12 now has a depth to it as parts of it are translucent.  

Stones 16 to 20:

Stones 21 to 25:

Stones 26 to 30:

Stones 31 to 35:

Stones 36 to 40:

The next step is Stage Six, the Pro-Polish tumble.

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 eighth Post in this series – the first Post can be found here.)

Stages Two, Three and Four tumble the stones in different grades of silicon carbide grit, shaping and smoothing them, ready for the two polishing stages. I use two different grades of tin oxide for my polishing – others use a very fine silicon carbide grit (e.g., 600 grade), sometimes skipping the 320 grit tumble stage, followed by only one polish-tumble with a powder such as tin oxide, aluminium oxide, cerium oxide, or chrome oxide.

I am following the stages set down in the Instructions provided by Colin Simmons of the Rotorua Rock and Gemstone Shop when I bought my tumblers. He also sells tin oxide powder, which is why I use it – it is the cheapest polish powder. The “Pre-Polish” stage uses powder of a five micron size while the “Pro-Polish” stage uses powder of one micron size. “Pre” and “Pro” look very similar so you have to be extra careful when labelling containers, sieves, beads etc. and when  using them.

Colin Simmons recommends that the Pre-Polish tumble should be for three to five days, which is shorter than the other tumble stages (which should be for at least seven days), and he states that the pre-polish mixture is retainable and reusable for about six batches of tumbling as long as it does not become contaminated.

Stage Five begins with gathering together the key ingredients: the 4lb tumbler, the white pre-polish tin oxide powder, a plastic shaker with a tight lid, and the plastic beads kept for the pre-polish tumbling:  

The 40 stones are then carefully placed into the barrel – care is taken to avoid chipping them. After the first layer of stones, I add a layer of plastic beads, just to make sure the stones are cushioned from the outset when tumbling starts:

After all the stones are in the barrel, I use plastic beads to fill the barrel up to about the two-thirds level (more beads are needed at this than at earlier stages as the stones are now smaller than when they started):

The next job is to add the pre-polish mixture. Often a previously used mix will be available. In this case, I created a new mix from the start. Following Colin Simmons’ Instructions, I place nine tablespoons of tin oxide pre-polish powder into the shaker:

The shaker is half-filled with water, the lid screwed on tightly, and the mixture is given a good shake (over the sink in case of leaks). I have found this to be the best way to mix up both pre-polish and pro-polish, having one shaker for pre-polish and one for pro-polish to avoid contamination:  

Colin Simmons states in his Instructions sheet that the resulting mix should have “the consistency of household paint”. I pour that into the barrel until it just covers the stones and the beads are floating:

The top can then be placed on the barrel and it can be put to tumbling:

As always, I record all of this in my tumbling log:

I try to tumble a pre-polish batch for a period of between five and seven days, even though Colin Simmons states that it is okay to tumble only for three days (as the minimum). In this case, I took the barrel off the machine in just under six days (five days 19 hours to be exact). I place the pre-polish shaker in a large shallow blue plastic bucket with the sieve over it (Note, January 2022 – I have just come across a suggestion from an experienced polisher that it is better using a plastic colander or sieve, not a metal one, to rinse stones at the polish stage as the metal ones tend to put metallic streaks on the stones. I am considering making this change):

Opening the barrel, I pour the contents into the sieve, allowing the pre-polish mix itself to drain into the shaker. Most of the liquid will end up in the shaker but if too much gets away, into the bottom of the bucket, then the bucket may need to be tipped up to pour it into the shaker:

I note on the shaker the date when the mixture was used. The pre-polish powder eventually settles at the bottom of the shaker. The mix can be re-shaken and used again, at least five or six times:

I then place the sieve  over my usual red bucket and wash the stones and beads with fresh water and put them back into the tumbler barrel for a soap tumble:

I use a small sieve (or my fingers) to recover any stray plastic beads that have escaped into the bucket, before discarding the water:

Once all the stones and beads are back in the barrel, I add water and a few gratings of sunlight soap, put the top on, and then start the barrel tumbling again. This washes both the stones and the barrel in preparation for the next stage, to avoid contamination:

I usually do a soap tumble for at least three hours, maybe as long as a day. In this case, I ran the soap tumble for just under 20 hours:

I wash the soapy water from the stones and plastic beads:

The water is a grey colour afterwards, and there are more escaped beads to recover:

I place the stones on a paper towel to dry prior to inspection and sorting in preparation for the pro-polish stage. They now retain a shine even when not wet:

I complete the tumbling log entry, adding the time for the soap tumbling. This brings Stage Five to completion. The next Post looks a little more closely at the stones at this Stage, comparing them with how they looked when collected.

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.

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.

The next Post is The Seven Stages in Tumble Polishing Stones: Stage Four, 320 Grit Tumble, 10-20 March 2018.

(This is the fourth Post in this series, the first one being here.)

Stage Two in tumble polishing involves tumbling rough stones in 100 grade silicon carbide grit. Stage Three involves tumbling in 220 grade grit. In between these stages, and in between later stages, I inspect each dry stone in a good light to check that it is suitable to go on to the next stage. Sometimes it may need to repeat a stage, or it may become apparent that there is a reason to discard a stone entirely.

The following presents the 40 Riverton stones after the 100 grit stage then describes their inspection and sorting. This initial stage of tumbling has resulted in smoother stones, and their colour is now more apparent.

Stones 1 to 5, dry then wet:

Stones 6 to 10:

Stones 11 to 15:

Stones 16 to 20:

Stones 21 to 25:

Stones 26 to 30:

Stones 31-35:

Stones 36-40:

INSPECTING AND SORTING

I inspected each stone when it was dry. I sat under a bright light and simply held the stone and turned it around. I found that 22 of the 40 stones are nice and smooth with no chips, nicks or pits. Some of these could skip the next stage and go straight to being tumbled in 320 grit but for the sake of this series of Posts I will keep them with the others for tumbling in 220 grit.  

Five stones (5,11,12,19 & 28) have very minor cracks, pits or slight gouges. Here are two examples: Stone 11 has a very minor superficial crack. 

Stone 19 has a couple of very small superficial cracks as well, one of which is indicated below.

Such very minor “imperfections” are usually easily removed in the 220 grit tumble. 

Eight stones (1,2,15,20,23,26,34 & 39) have small or medium-sized pits or cracks. These also usually disappear after tumbling in 220 grit. Three examples: Stone 1 has a number of small pits.

Stone 15 has a small gouge in the side. 

Stone 20 has a small surface crack.

Three stones have issues that I would normally hesitate over for some time, considering whether to tumble them again in 100 grit. Stone 4 has a small indentation in its side.

Stone 25 has a number of noticeable cracks and pits in it. It is one of the smaller stones. 

Another small stone, Stone 31, has some chips on it. It is easier to get rid of such chips if the stone is larger because it can stand up to losing another 5% or more by repeating the 100 grit tumble. But because it is so small, it can be worth trying the 220 grit tumble first to see if it improves without losing too much of its mass.

Finally, there are two stones that I would normally put back to another 100 grit tumble. Stone 9 has a medium-sized pit in its side.

Stone 10 has an even deeper hole in it from where a small embedded piece of stone has loosened and come out.

All 40 stones will be retained for the 220 grit tumble (Stage Three) so that we can monitor what happens to them – see The Seven Stages in Tumble Polishing Stones: Stage Three, 220 Grit Tumble, 4-13 December 2017.