What’s the difference between disk method, washer method and shell method in calculus?
Disk methods, like that used for coffee beans and table salt, are difficult to use for larger quantities of calcite. The wetting agent doesn’t work well over a large surface area, so the solution wets only an outer rim around the disk and not enough of the material itself. If you try it with sugar granules then you’ll see what I mean – they just won’t dissolve at all. But it’s worth noting that disk methods can also work very well where small volumes are required or when coating one side of the sample to measure half thickness by comparison against a flat target.
The washer method is another way of measuring half thickness – in this case a small disc, or ‘washer’ is used. This may sound like a good idea but there are many practical issues when using it in actual applications.
First consider the geometry. We need to know the radius of the washer and its thickness. If one of these is fixed then the other has to vary. If you measure the radius first, then it’s difficult to know what thickness to set for the washer because this also depends on how hard you press down when measuring (to get a good contact area). If you choose too loose a fit then the washer will be poorly compressed and give you a false reading; too tight and the washer may become deformed. Making multiple measurements with different washers of known thickness doesn’t help because you don’t know how much the washer has deformed during use (and it will definitely deform on some compression paths).
Shell method is another way to measure half thickness. By double diffraction, it can avoid the effects of compression. But for this method, you need to measure two reflections simultaneously that are safe to use the same aperture angle. The first step is to mark a target area on your sample which should be flat or at least level with the surrounding material. Since calcite has cleavage planes then any surface scratch, gouge or cut is unacceptable because it will encourage preferential growth in that direction.
