In this month’s blog I’m going to explain two techniques for hand grinding your workpieces. Yes, they are relatively slow, but the results definitely make them worth your considering.
You can buy them as pre-cut, synthetic aluminium oxide, or occasionally silicon carbide grinding sticks, commonly 6 mm square by 150 mm long. But they are much more economical when you buy the blocks which measure around 25 x 60 x 200 mm and cut them down as previously mentioned in my diamond blades blog – "sharpening" your blades as you do so. The blocks are composed of either a single grade, or two grainsizes.
Waterstone blocks and sticks are offered by most carving suppliers, and you can also find a selection in some engineering supply outlets - and of course, on the internet. And you may come across unmarked blocks for a good price at a car boot sale or in a second-hand shop – experience will tell you if you need that grainsize. When you get it home you can settle down and compare the grainsize with those of known sticks – and then try it out for a direct comparison.
You can occasionally find Arkansas stone in stores. It’s a natural stone which comes in a range of grainsizes - the harder it is, the finer the grainsize, I’m told. But they tend to be quite expensive. There are also oilstones, but they can be a bit messy and won’t develop a good slurry when used with water.
Oil- & waterstones were originally used by tool and die makers to remove burrs from machine tools and dies, and when called whetstones and used dry, for sharpening knives and other blades. Where a clean, sharp edge was critical, these beauties did the job! And now we carvers can use them to great effect!
As mentioned above, they come in a range of grain sizes. The coarse, shape-forming grades are the 120 - 320 grits. The finer grades for cleaning up your work include (somewhat arbitrarily) 400 - 900 grits. And then there are the pre-polishing grades of 1000 – 8000.
For any grainsize, the stick itself can be relatively harder or softer and the grains sharper or duller. These factors affect how a stick will perform.
This is borne out by a recent experience I had where I have ignored the manufacturer's description of some sticks being 600 because even if the grainsize is correct, they behave like my 400, and leave deep scratches that I then have to remove with other more gentle 600 sticks. You sometimes need to be flexible in your categorising.
And I once received an order of 120 & 240 grit sticks that were clearly mislabelled, despite the company denying it. All I can say is if a 240 grit stick is more aggressive (leaves coarser scratches) than a 120 then there is something wrong! They work fine my way around!
When grinding a workpiece with a particular grainsized stick, you won’t just use one. You will develop a number of useful shapes. The smaller the work, the finer the sticks you will use.
As you finish with one grainsize, you need to choose sticks with roughly double the number of the current grade – so you start with 120, then go on to 240. I tend to then go to 400 from there, but many people jump to 600, then move on to 1000, followed by 6000 (by this time the difference in grit size is minimal and you’d be wasting your time by taking an intermediate, 2 – 3000 step).
But don’t assume all nephrite polishes the same. Each piece of stone is different and requires an individually tailored process. Some produce a great polish quite easily, others really make you work! It’s all about the crystal size/s of the rock you are polishing, as well as the structure, hardness and chemistry of the individual crystals involved. It keeps you on your toes!
So, to begin, you soak the stick in water for a few minutes (except the pre-polishing sticks as they don’t absorb water), and then set to work. As you work on a particular piece with long, slow, light, even movements, it will be ground away. But the grinding stick will also be ground away, which between them, creates a slurry. This holds the free grains in suspension to aid grinding and helps to keep the stick clean of clogging particles.
Don’t press too hard or you exacerbate possible undercut and also run a really good chance of breaking the stick.
I check my work regularly to see when a particular face is ready for the next stage of grinding. It depends on you, but I don’t think tool-marks are attractive and do everything I can to remove them from my work. Tool-marks are scratches and grooves made in the earlier stages of working on a piece (say from your saw blade, the coarse diamond belt or from any of the coarser sticks, which, due to oversight, remain on the surface of the piece). And as you use finer grades of abrasive, they cut progressively less aggressively, and deep scratches are likely to remain in full sight and spoil otherwise lovely work.
The best way to avoid this is to apply your sticks at right angles to the previous cut, and to check the work regularly with a loop or hand-lens to make sure that all of the previous scratches have been removed before you begin to grind perpendicularly to the previous direction of grinding. The best way to get rid of a scratch in a particular direction is to grind across it until it is gone. This cross-wise grinding also evens out the curves. It can be time-consuming work, but it’s worth it.
Now of course you don’t always have to begin grinding using 120 grade sticks. If you use power tools such as a 600 grit diamond belt, then your first stick could be 1000 grit. Or there might be some areas that need to be cleaned up and you go back to 400 on them, on to 600 and then progress to 1000. Or if you have produced a good rough finish with some other tools, you may be able to start using 400 or 600 grit. It’s all about saving yourself time and energy – if there is a bad scratch on your work, go back to coarse, say 320 or 400, and then go through the finer grades rather than spending an absolute age by persevering with 600. You’ll save time in the long-run.
Cleanliness is paramount with grinding sticks! Keep each grade separate from others, such as I do in the picture. If in doubt of cross-contamination, wash the sticks under running water and scrub them with an old toothbrush. And when you are using them, you will need to change the water, and clean your bench & rest each time you change the grade.
Depending on the shape of the workpiece I am grinding, I find it is useful to have some really small sticks – maybe only 3 mm square by around 100 mm long. Round section sticks are less easy to produce and sometimes hard to find. But as a resourceful carver, you’ll find a way!
Sticks are good for grinding complex shapes, and cleaning up complex detail on a piece.
The benefits of using grinding sticks are:
The disadvantage of using grinding sticks is simply that the process is quite slow.
Diamond files are typically small, or “needle” files, and are a direct competitor to coarse grinding sticks (please see Picture 2, posted above, in September). The files are made of steel and are coated in 150 – 240 grit diamonds. You can buy them in sets including flat, square, triangular, half-round & round and they can be parallel and/or tapering.
Files are very useful when you need to achieve a definite result with a piece, or are working on a pair of pieces, for instance cufflinks or earrings, and they have to be the same shape & size. With a file you can control the amount of grinding quite accurately, whereas with a grinding stick you are going to round off the edge and may end up with wear in places you don't want it.
Like grinding sticks, files are also very useful in removing the tool-marks and ridges from the rough forming stage.
Much of what I’ve written for grinding sticks, above, can also be applied here:
So, however you get through the forming and pre-polishing stages, the next step is polishing, which I’ll be writing about shortly! Happy carving!
On this page I intend to add monthly updates on aspects of jade carving. I also plan to invite more experienced carvers to offer a "master-class" on a particular subject of their choice. With this I hope to enthuse both the novice and the expert in this ancient and beautiful art-form/craft. And comments are welcome!