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

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). 

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Sieves for the different grit and polish grades

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.

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Taking the lid off the 320 grit tumble.
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Washing the stones after the 320 soap tumble.
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The 40 Riverston stones fresh out of the soap tumble folloing the 320 grit tumble.
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The amount of plastic beads used with the 320 tumble.
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The water from the post-320 soap tumble – significant additional slurry is removed from the stones and barrel.

Once the stones are dry I put them on the scales and discover they weigh 1261 grams.

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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.

South Island Stone Collecting Trip, Part Two – Riverton to Kaikoura

Continued from Part One.

Next stop was Riverton at the very bottom of the South Island, where we were based for the next three weeks – many thanks to Helen and Ray for the generous use of their holiday home. We collected many beach stones from the Riverton area and from nearby Gemstone Beach at Orepuki. The weather was cool and windy at times but not bad enough to discourage stone collecting. We met a handful of fellow stone collectors at Gemstone Beach and exchanged greetings and stories. We also spotted dolphins swimming off the beaches at Riverton.  

While based at Riverton, we took a day trip eastwards to Waipapa Point, Slope Point and Curio Bay, the latter being well known for its petrified forest that is uncovered at each low tide.

One day we walked the Long Hilly Track through an old gold mining area near Orepuki. This included part of the 40 kilometre long Port’s water race, built in the 1870s and 1880s with the help of Chinese miners. We visited the Riverton Museum and found an excellent display on the Chinese goldminers. On a visit to the Southland Museum in Invercargill, we saw a natural history room that included a lot of local geological displays and information.

On the trip north, on the way home, we stopped off to see the Moeraki Boulders in North Otago. Some sea mist came down even though it was the middle of the day. The boulders are large spherical rocks, concretions that have been exposed through shoreline erosion from coastal cliffs. They consist of mud, fine silt and clay, cemented by calcite. The degree of cementation varies from being relatively weak in the interior of a boulder to quite hard at its outside rim. The boulders are cracked and eventually fall apart after having been exposed for some time. 

A day was spent at Birdlings Flat, near Christchurch, where we collected quite a few stones for polishing. We walked to the point where the stony beach meets the volcanic mass of Banks Peninsula, briefly disturbing a resting seal. The tide was low enough for us to look at the stones in the small bay past the seal. 

We also took the opportunity to visit Akaroa on Banks Peninsula where we saw thick clouds rolling slowly down over the hills.

The road further up the east coast of the South Island, through Kaikoura, was open – it had to be rebuilt after the November 2016 magnitude 7.8 earthquake as well as additional landslides caused by recent storms. So we were able to view the earthquake aftermath, including the land that been raised out of the sea. Some parts of the coast were uplifted by six metres. One of the places we visited on the Kaikoura peninsula was Point Kean, well-known for its seal colony. A large area of many hectares/acres now lies dry where it once was under the sea. 

Then it was home across Cook Strait, a choppy but not uncomfortable crossing.

 

South Island Stone Collecting Trip, Part One – Oparara Basin to Lake Tekapo

I have recently returned from a few weeks in the South Island of New Zealand, travelling by car, collecting stones from places like Charleston, Riverton, Orepuki and Birdlings Flat and visiting interesting landscapes like the limestone caves and arches of the Oparara Basin (north of Karamea), Arthurs Pass, Lakes Pukaki and Tekapo, Curio Bay, Waipapa Point, the Moeraki boulders, and the post-earthquake Kaikoura coast. 

One of the first places we stayed at was Karamea in the north Buller region of the West Coast. From there we drove the narrow gravel road north to the Oparara Basin and walked to the Oparara Arch, Moria Gate and other limestone caves and landforms. 

I collected a small number of limestone stones from the Oparara River – they appear to have fossilised shells in them.

Going south along the West Coast, beyond Westport is the town of Charleston and its small twin bays, Constant Bay and Joyce Bay. Constant Bay was named after the ketch “Constant” whose Captain, Charles Bonner, in the mid 1860s managed to squeeze the boat into into the tiny bay, bringing supplies for the town that had sprung up due to the discovery of gold. The rocks that line the shore of these bays have large veins of quartz with masses of mica in them. At the northern end of Joyce Bay we came across of pocket of stones and rocks from a vein that must have been almost pure mica. We chose one small boulder-sized rock to bring home.

There is so much mica in the rocks around Constant and Joyce Bays that the sea sparkles with it.

One hundred kilometres south of Charleston is Kumara, where we undertook a short walk to see Londonderry Rock.  This enormous 4000 tonne glacial erratic was dislodged by miners while sluicing for gold in the early 1880s. As noted on the information panel at the start of the walk, local legend has it that when it was dislodged from its position it caused earthquake-like shudders that stopped the clock in the post office! 

Driving across Arthur’s Pass, we passed Castle Hill, Omarama and Lakes Pukaki and Tekapo.

Continued in Part Two.