GB2202225A

GB2202225A - Cement for use in working gemstones

Info

Publication number: GB2202225A
Application number: GB08803192A
Authority: GB
Inventors: Alec Leibowitz
Original assignee: BRILCUT PATENT; Brilcut Patentanstalt
Current assignee: BRILCUT PATENT; Brilcut Patentanstalt
Priority date: 1987-02-11
Filing date: 1988-02-11
Publication date: 1988-09-21
Also published as: GB8803192D0; GB8703075D0; ZA88923B

Abstract

In order to enable a gemstone, e.g. a diamond, to be centered on a dop for bruting, the gemstone is held on the dop using a cement which is a self-hardening liquid adhesive incorporating a fine mesh size filler which is wettable by the adhesive. The cement preferably comprises an epoxy adhesive and the filler may be alumina, silicon carbide or glass. The gemstone is centered before the cement sets, and the setting of the cement may be accelerated by heating. After bruting, the gemstone can be removed by heating the dop and stone to 150 DEG C.

Description

CEMENT Many cements are known for cementing a gemstone onto a holder (referred to hereafter as a dop) to enable the stone to be worked. The cement must give the operator the possibility of seating the stone firmly on the dop without any tilting of the stone. The cement must have high strength and good impact resistance, and also be releasable in a simple procedure to free the stone from the dop.

Although other arrangements have been proposed, normal practice is to heat the dop, place on a small amount of cement which melts on heating, position the stone on the dop. and allow the cement to cool and solidify.

Considerable skill is required in placing the stone on the hot dop even to an approximation of a central position. The stones would normally be sawn halves.

which have a flat sawn face (but having parallel lines on it due to the sawing), but a very irregular edge for the remainder of the stone. This stone should be positioned to an accuracy of greater than 0.025 mm (1 thou). Furthermore, due to the speed with which one should work, there is usually a significant thickness of cement between the stone and the dop - note that the stone may have a total height of say 0.5 mm and that a 0.1 mm thick layer of adhesive alters the position of the sawn face by 20%, which can be extremely important for automatic machinery such as bruting machines. A further severe disadvantage of a significant thickness of adhesive is that the stone can tilt with respect to the dop if the adhesive cools non-uniformly.

The present invention provides cements as set forth in Claims 1 or 13 and methods as set forth in Claims 14, 17 or 18. The remaining claims set forth optional features of the invention.

The cement as set forth in the Example below has been found to be very effective, and it is believed to be effective because the fine mesh filler is very hard and enables the adhesive to withstand high loads without distortion due to the blocking effect of the particles of filler in the matrix of adhesive. Thus the filler should be of a hard material, such as glass, e.g.

crushed glass, and is preferably an abrasive, particularly an abrasive having a hardness comparable to that of gemstones. The filler prefrably does not have a high coefficient of expansion, and thus there is less danger of the cement loosening due to changes in temperature of the dop or of the stone.

The adhesive can be any self-hardening, i.e.

self-curing, adhesive. A cold-setting adhesive can be used. even though setting may be accelerated by heating. The preferred adhesive is an epoxy adhesive which is supplied as two components, the adhesive itself and the hardener, and it is desirable for both these components to be in liquid form. In order to ensure that the stone does not drop off the dop, particularly when centering the stone on the dop, the cement should be sticky or tacky, i.e. viscous, when cold. Epoxy adhesives have this property. Epoxy adhesives have another property which is very valuable. At a curing temperature, they become very runny, i.e. of very low viscosity.This has the important advantage that the stone tends to seat very firmly on the dop, with perhaps only one grain thickness of adhesive between it and the dop, at least while there are ridges caused by the sawing (there may be somewhat thicker cement in the grooves caused by the sawing), due to the adhesive forming only a very thin layer on the grains.

Furthermore, epoxy adhesives degenerate when heated to 1500C, and the dop and stone can be heated to this temperature in order to release the stone from the dop after working.

The amount of filler should be chosen to enable the cement to be very plastic when applied, and not too putty-like. The grain size of the filler should be fine in order to avoid gritty particles spacing the stone excessively from the dop.

Example A two-component cement was made up as follows (parts by weight): First Component: 1 part "Araldite (RTM) normal (not rapid) epoxy adhesive; 1.5 parts Crystallon", a proprietary ground, crystallised (not precipitated) alumina, average mesh size 600.

Second Component: 1 part "Araldite" hardener. for the above adhesive; 1.3 parts "Crystallon", as above.

Both components have a similarly tacky or sticky consistency.

The mesh size of 600 is in accordance with the manufacturer's designation. This is believed to be a U.S. mesh size such as U.S. Standard or U.S.T.M.

Standard, but could be Tyler or British Standard. The mesh sizes quoted herein are in accordance with the manufacturerls designation for "Crystallon".

A spot of the cement is applied to the top of a dop and a diamond sawn half is placed with its flat surface on the top of the dop, and pressed on. The diamond is centred for bruting in any suitable way. The cement will remain fluid enough for centering for about one and a half or two hours. The dop with the diamond and cement is then heated to 70 C. This causes the cement to become runny, with the adhesive itself very runny.

However, the adhesive is quickly cured, and is fully hard after one hour (alternatively 500C could be used for three hours or 100 C for ten to twenty minutes).

The diamond is bruted in a bruting machine and the dop and diamond are then heated to 1500C, which causes the adhesive to degenerate and the diamond to come away from the dop.

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention. The invention also consists in any individual features described herein or any combination of such features or any generalisation of such features or combination.

Claims

1. A cement for cementing a gemstone onto a holder to enable the stone to be worked, the cement comprising a self-hardening liquid adhesive incorporating a fine mesh size filler which is wettable by the adhesive.

2. The cement of Claim 1, wherein the filler is of average mesh size of 350 to 1000.

3. The cement of Claim 1, wherein the adhesive is of average mesh size of about 600.

4. The cement of any of the preceding Claims, wherein the adhesive is a two-component adhesive, at least one of which is liquid.

5. The cement of Claim 4, wherein the adhesive is an epoxy adhesive.

6. The cement of any of the preceding Claims, wherein the filler is an abrasive.

7.The cement of any of Claims 1 to 5, wherein the filler is alumina.

8. The cement of Claim 7, wherein the abrasive is ground, crystalline alumina.

9. The cement of any of Claims 1 to 5, wherein the filler is silicon carbide.

10. The cement of any of Claims 1 to 5, wherein the filler is glass.

11. The cement of any-of the preceding Claims, and being sticky when cold.

12. The cement of any of the preceding Claims, which becomes substantially less viscous when heated to a curing temperature.

13. A cement for cementing a gemstone onto a holder to enable the stone to be worked, and substantially as herein described in the foregoing Example.

14. A method of working a gemstone, comprising using the cement of any of Claims 1 to 11 and 13 to cement the stone onto a holder for holding the stone during working.

15. The method of Claim 13, and used for bruting the stone, the cement being applied between the stone and the holder. the gemstone then being centered on the holder, and the cement then hardening.

16. The method of Claim 12 or 13, wherein after applying the cement, the cement is heated to a curing temperature, the cement becoming substantially less viscous when so heated.

17. A method of working a gemstone, comprising cementing the stone onto a holder using a self-hardening, viscous adhesive which, when heated to a curing temperature, becomes substantially less viscous, heating the holder and stone to the curing temperature, the adhesive initially becoming substantially less viscous and then hardening, and using the holder to hold the stone during working.

18. A method of cementing a gemstone onto a holder and working the stone, substantially as herein described in the foregoing Example.