Good morning all has anyone ever seen (polishing gem stones on a CD) I came across it on YouTube while looking for something else maybe not the norm but might be worth a look
cheers
Certainly polishing with diamond can be done on Perspex, or other polymer laps, but you would expect rounding of facet edges. If you are having problems with digging in to tin laps when polishing low angle facets, it sounds as though you may be trying to polish on the "leading" side of the lap, rather than staying on the "trailing" side. This can be a big problem on some machines that have the cutting head free sliding on the mast (e.g. Graves), where the stone will dig in and push the head unit up the mast. Also if a machine is getting a bit old and sloppy, there is more tendency to dig in.
So far this trial is going well. And tests show that perspex/ plexiglas/ lucite call it what you will - is doing a great job on the pre polish phase. From a commercial point of view or cutting some quick gems that the wife wants to wear you could get a pretty high polish on a 2.5 micron 4000 grit paste. But best to use a finer micron diamond pastes etc if you decide to cut for comps. Mind you I'm adding a small amount polishing compounds used in the aviation industry to keep that perspex disc smooth and shiney at the same time. Still testing sunscreen as a extender also, which seems to be better than a few other extenders on perspex.
As perspex has a moh hardness similar to copper, around 3 there is no logical reason why it shouldn't make a better alternative. As it's easier to clean and creates no metallic residues.
As for the moh hardness of tin and lead, both are similar and very soft at 1.5 on the scale, leaves plenty of metallic residues that are not too friendly, and require products like WD40 or similar products to clean the black residues off. Same goes for the cooper laps. Makes you wonder about the compositional values of both tin and lead being the same hardness and what real benefits are gained as a alloy. As for Nickel it has a moh hardness of 4 making for other alternatives. I think the Batt Laps have a harder composition although I've never owned one, it's said to polish a sharper edged facet with less rounding. Hard to imagine that a softer composite such as tin lap should polish a sharper less rounded facet than a harder composite. Although I have heard from other faceters that the UItra Lap Spectra laps are great polishing quartz, but leave a more rounded facet. But that's using impregnated cerium, and is used mainly for gems such as quartz with a lower moh. And I suspect the thin plastic polishing film would have a softer composition than perspex.
Anyway, I will be doing are few other trials. And may even do a test sample on a gem by polishing a set of facets with a tin lap with diamond, and a set of facets on a perspex lap with diamond and have a it evaluated by myself under a 10X loupe and by fellow gem cutters at a future meet to see if there is a real difference, or a mindset within the hobby.
I've included a photo of the pre polish on the CZ using 4000 grit diamond on perspex with perspex polishing compounds.
Just a few comments. Firstly while lead residues are undesirable, tin is a very safe material (used for food contact)
Second, there is much more than hardness considerations in determining what makes a good polishing lap. Microscopic grain structure and porosity for example. In the case of tin and its alloys there is, I believe, the effect of oxidation to form a thin film of tin oxide on the surface - this combined with 50k diamond could be expected to give a great polish.
In the case of porosity, the "special alloy" that I use now is a tin alloy with a surface that, viewed under high magnification is extremely porous - obvious to see why it would work so well.
Good point you make Bruce with the use of science. In fact I was thinking of faceting a Portuguese cut polishing one step with a tin lap and and the other step with a perspex lap and so forth. Being that I would be the only person knowing the polishing order sequence, then see if those at the my gem club can detect any variations. And to go one step further, I'll ask a close friend of mine currently completing a doctorate in nanotechnology at Melbourne Uni to check the facets under a high powered microscope and get him in percentage terms evaluate the sharpness of the facets. Of course there may be a few other variables involved, but it would make for interesting information.
I think checking under a microscope a great idea, a little scientific objectivity may help our understanding of how certain materials behave during the polishing process.
My theory of facet edge rounding with lucite is based on material in Broadfoot & Collins, the slurry applied to the lap rounds the edges of the facet.
Lucite is relatively resistant to compression so you would not expect it to bow at the facet face causing rounding.
regards
Gordon
It will be interesting to see results of tests. Seems that with cerium, it has an effect on rounding of facets, mostly on Ultra Spectra Laps. Bruce mentions the porous nature of tin being a benefit and the oxidation of tin being a benefit. I can see a benefit of tin oxide, yet I'm not convinced by the analysis of the porosity of the tin, as Pb0 (lead oxide) acts as a sealant, that's why it was used in paints. And mixed with a extender you would expect it to counteract the porosity. However, without conclusive evidence it's hard to determine whether porosity plays a major benefit of polishing or not. As this is a test only on CZ, further tests on other gems may prove that tin laps are more beneficial. A huge advantage I can see with perspex laps it that you can use both sides. So you can use different gradients of diamond pastes without too much risk of contamination. Also, they are cheap.
You have misread what I said. The porosity I referred to was in "Special alloy" - that is a tin alloy, but does not contain lead. My reference to tin oxide also referred to tin laps - no reference to lead. I have never used laps containing lead. Pure tin, non lead pewter, etc.
If pores were sealed with any oxide, however, one would expect that sealer to be quite soft and still readily able to accept hard diamond particles.
Anonymous wrote: You have misread what I said. The porosity I referred to was in "Special alloy" - that is a tin alloy, but does not contain lead. My reference to tin oxide also referred to tin laps - no reference to lead. I have never used laps containing lead. Pure tin, non lead pewter, etc. If pores were sealed with any oxide, however, one would expect that sealer to be quite soft and still readily able to accept hard diamond particles. OK.....So since you use a "Special Alloy" lap, what is the other metal component in it? That's assuming that the reader assumes one component is already tin? We are moving away from the main topic what this post is really about!
OK.....So since you use a "Special Alloy" lap, what is the other metal component in it? That's assuming that the reader assumes one component is already tin? We are moving away from the main topic what this post is really about!
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