Keeping sharp
The need for a sharpening stone sends this writer off on an interesting quest
Iwanted a natural sharpening stone and didn’t want to pay for a Japanese water stone, many varieties of which I have seen for sale in Japan. Some of these cost more than $100. Arkansas and Washita stones from America are similarly pricey.
I came across a reference to Māori using stone from Lyttelton Harbour for sharpening and I thought that I could collect a stone from their old quarry, shape and flatten it, and I would be in business — and my cash would stay safely in my wallet. It was while I was trying to get more information about local sharpening stones that I was struck by the similarities between sharpening steel and stone tools. Both use abrasives to wear away two flat surfaces that meet at the cutting edge.
First Kiwi-made tools
For the early Polynesians, arriving at a new, previously uninhabited Pacific island would have been like being given the keys to Pak ’n’ Save. Large flightless birds and plentiful sea mammals, with no fear of humans, would have provided easy eating for generations. Because of the typically very long voyages undertaken to find new land, replacing stone tools as they wore out or broke was not so easy. They had to do it themselves using the materials at hand. In the South Pacific and Hawaii the best material was the volcanic rock, basalt, of which the islands were composed. When Māori arrived in Aotearoa 800 years ago, they would have been evaluating the stone outcrops’ potential for toolmaking and tool sharpening from the very first.
Obsidian — a perfect knife
It is thought that these first arrivals to New Zealand were very accomplished sailors, so they had the mobility to quickly explore their new home and identify suitable toolmaking stones in many, especially coastal, places.
They soon discovered the volcanic glass obsidian, called ‘matā’ by the Māori, at several sites — the best came from Mayor Island in the Bay of Plenty. Obsidian was valued because fragments naturally have very sharp edges, which make them useful as knives. Larger blades for the very important woodworking adzes were hard to make from obsidian and many different stones were used for this purpose. The finegrained basalt from the Coromandel region was preferred in the early days of Māori settlement. Later, the hard, grey sandstone greywacke, of which the Southern Alps are made, was most commonly used, although the supreme nephrite — pounamu — from the Arahura and Taramakau Rivers on the West Coast of the South Island, was most valued.
The need for sharpening
Adzes (toki) and chisels (whao), whatever they are made from, have to be sharp to cut wood. Māori used either a mixture of sharp sand and water on a hard, flat stone, or a flat piece of wetted
sandstone, to sharpen and polish stone tools. Evidence of the use of sandstone outcrops for this purpose can still be seen at some locations, usually very near to water. Sand and sandstone are both a result of the weathering of quartz. The small, hard, sharp-edged silica crystals, made by the break-up of lumps of quartz, scratch the stone being sharpened, abrading it away. The final polish was achieved by using red ochre (kokowai), which contains rouge, applied with, perhaps, animal skin. Traces of the red, very fine abrasive can be seen on some toki.
Whatever they are made from, [they] have to be sharp to cut wood
Steel arrives
Contact with European sailors resulted in major changes to the tools of the Māori long before any significant impact was felt in other aspects of traditional Māori life. Carvers made increasing use of steel chisels made from nails. Adze blades were made first from barrel hoops, then from cutters from carpenter’s planes, and finally were largely replaced by European adzes and axes.
The stone adzes were then used for gardening; abandoned; or, if pounamu, reshaped into hei tiki. Stone adze heads are continually being discovered and handed into our local museums.
These new metal tools were sought after by Māori because metal makes a more durable cutting blade that holds an edge better and is probably sharper too, but sharpened in the same way as stone blades.
The cutting edge of a metal tool is also formed by the grinding of two surfaces that meet at a line. Again, small, hard, sharp crystals are used to scratch the metal to make the flat surfaces. The harder the crystals, the harder the metal that can be sharpened. The smaller the crystals, the finer the scratches and the sharper the edge. The larger the crystals, the faster the edge is sharpened.
Hard, crystalline materials
When we look at a list of hard crystalline materials, we see many of the substances that we use for sharpening in the shed. Diamond, silicon carbide (carborundum), silica (sand), aluminium oxide (alumina), rouge (iron III oxide), and glass are all used as abrasives and all sharpen in the same way as the sand slurries of pre-European Māori.
Before the last century water stones were the usual sharpening material. These were made of sandstone, which is a sedimentary rock consisting of silica crystals that have been bonded together by heat and pressure deep beneath the Earth’s surface. The hard silica crystals on the face of a water stone removed metal from the surface being sharpened and plenty of water kept the metal cool and washed away the mixture of lost metal particles and detached, broken silica crystals called ‘swarf’. This allowed new sharp crystals to be uncovered and brought into action. As the stones were used they were worn away and stopped being flat and so had to be flattened using a flat, hard rock and a sand slurry — just as the Māori did.
Water stones revered
The best water stones were highly valued, as they continue to be today, and were traded widely around the world. The first Europeans to settle in New Zealand would have brought a water stone in their swag as well as the classic axe and a Bible. If they were English the stone would have probably been a Charnwood Forest stone from Leicestershire, but there would have been a variety to choose from — some,
such as Turkey stones, from as far away as Lebanon and Syria. Māori called water stones ‘hōanga’, and an example of these was a sandstone from tiny King Billy Island in the South Island’s Lyttelton Harbour. Some hōanga were so revered that they were named and had stories composed about their history. One famous example near Rotorua was said to have been brought from Polynesia on the Arawa waka.
Finer abrasive oil stones
Until recently, oil stones, consisting of silicon carbide crystals of a uniform size embedded in a bonding agent, were universally used by workers in Western countries to keep a sharp edge on tools. Being manufactured, oil stones were relatively cheap and consistent. The ‘oil’ in the name refers to the light
mineral oil which was used on the stone to lubricate and cool the abrasive and to allow swarf to be moved away from the sharpening surface. Oil stones are also commonly made from aluminium oxide (alumina), which has generally finer abrasive particles and so produces a sharper edge more slowly. These are called India stones. Some natural stones, such as Arkansas stone from America, are also used as oil stones but are much more expensive.
Diamond days
The serious woodworker of today would almost certainly use diamond as a sharpening material. Diamond is so hard that it is worn away by sharpening much more slowly than other abrasives, so only one layer of diamond is used. This is bonded to a (hopefully) flat surface that incorporates depressions to remove swarf so that the diamond crystals aren’t clogged with metal particles. Using the plate wet apparently makes it last longer.
The diamonds used are relatively inexpensive because they are manufactured. They are sorted by size so that diamond plates of various coarseness, although not very fine, can be produced. Diamond plates are unrivalled in removing metal quickly and in sharpening very hard alloys like highspeed steel, but struggle to produce a mirror finish on the tool and hence a really sharp edge. Diamonds are also excellent conductors of heat, so the surface being sharpened shouldn’t overheat and lose temper. ‘Temper’ is the hardness caused by the heat treatment of steel.
Japanese water stones
Japanese tools are increasingly being used by both professional and amateur woodworkers, as are Japanese water stones. These have as abrasives the familiar silica particles, but their difference is that the particles are embedded in a clay matrix. This makes the stone softer than sandstones with the advantage that new sharpening particles are continually being uncovered as a tool is moved over the stone, leading to rapid sharpening.
The disadvantage is that the stone wears quickly and has to be frequently flattened (or ‘trued’) to maintain a flat sharpening surface. Diamond plates would be suitable for this, or specialized flattening surfaces used with silicon carbide powder could be used. Japanese water stones were originally natural, being quarried at numerous locations in Japan, but are now mainly manufactured, several companies producing various brands. Like other sharpening stones they come in a variety of abrasive particle sizes, some of which are very fine, perhaps the finest available. Finer particles equal sharper edges but slower sharpening. Overall Japanese water stones provide a sharper edge for less sharpening effort, which means that they can be sold at a premium price.
The old name for sharpening
As a child I used to see a council worker regularly cut the grass verge of our road with a scythe. Every 10 minutes or so he would take a round oil stone out of a leather pouch attached to his belt and wipe it a few times along the cutting edge. This process was called ‘whetting’, the old name for sharpening, and he would have been said to have ‘whet the edge’ of the scythe. The oil stone would have been called a ‘whetstone’. He would have periodically oiled the stone’s surface with an oil-soaked rag from the pouch. I can’t remember him having an oil can concealed about his person; perhaps it was in another belt pouch. Or was it in his … Truck? Unlikely. Bicycle? Horse? It’s a long time ago.
I was aware that getting a sharpening stone from tiny King Billy Island in Lyttelton Harbour would be problematic because it is administered by the Department of Conservation (DOC) as part of the conservation estate. Without much hope of success I contacted DOC, the local iwi, Te Hapu¯ o Nga¯ ti Wheke, and the Otamahua / Quail Island Ecological Restoration Trust, who are all stakeholders in the island. The DOC’s position was that nothing should be taken from conservation land without permission, unless I had a prospecting licence. So that was that. Interestingly King Billy Island is known as ‘Aua’ to the Ma¯ ori. Another meaning of ‘aua’ is “I don’t know”. A situation reminiscent of the Indigenous Australian word ‘kangaroo’, which has the same meaning.
I rang the Geology Department at the University of Canterbury and talked to one of the professors. He told me that there were are other sandstone outcrops in the ancient volcano caldera that forms Lyttelton Harbour. These are the remains of the seabed that the volcano thrust through millions of years ago. One is at nearby Orton Bradley Park, which is owned by a charitable trust. I applied to the park manager to get a small block of the sandstone, to which he was agreeable. On the day, we trudged through the rain and mist to the quite extensive, well-fenced quarry which was last worked more than 40 years ago. The manager thought it was unwise to approach the workings too closely, especially as water was running down the rock face and a rock fall was not out of the question. There was plenty of sandstone rubble among the gorse bushes below the quarry face, and he very kindly gave me a plate-sized piece. We discussed whether Mr Bradley, the original owner, would have used the sandstone to make grinding wheels such as the ones in the old sawmill on the property.
The manager thought that the grindstones were probably English, but he pointed out that one of them, powered by the overshot water wheel, had ‘1900’ carved into its side. Would you do that to a purchased grindstone, or would the rough sides of the grindstone indicate that it had been made on the property with stone from their quarry?
The animal-welfare organization where my younger daughter worked before she went back to university has a bench-top manufacturing business behind it owned by Shane Boyd. For a small fee Shane cut the Orton Bradley Park stone into three pieces with the 450mm water-cooled diamond blade of his Italian computer numerical control (CNC) bench-top machine. This produced a 200x100mm sharpening area. The surface came up well, with a creamy colour with red-brown flecks. I used water as a lubricant, just like the Orton Bradley Park grindstone, as I slowly sharpened a chisel that I found at an op shop to a near-mirror finish. Result! A big thank you goes out to Clive Fugill, Vicki Blyth, and Lisa McDonald from Canterbury Museum, as well as the staff at the beautiful Orton Bradley Park for their help in the writing of this article.