Method of coloring cut gemstones

The invention relates to a method for coloring polished gemstones by introducing metals or metal oxides into a surface layer by means of heat treatment.

The superficial coloring of gemstones with transition metals is a technique that has been described again and again since ancient times. In recent times, efforts have been made to lower the temperature during the heat treatment in order to protect the surface of the stones. This was achieved, for example, in that the ground stones were vapor-deposited not with cobalt oxide alone, but with cobalt in combination with aluminum oxide (cf. DE-A 22 29 909). From Balitskii (cf. No. 3,950,596) it was stated that one does not have to apply all oxides to a polished gem stone, but can use the stone itself as one of the reactants. The surface can be colored either by diffusion or by reaction.

A number of important minerals that are used for gemstones are suitable for coloring. In addition to corundum, these include various spinels, the island silicates olivine, zircon, topaz and garnet, the aluminosilicates such as andalusite, thistle or mullite, as well as the ring silicates beryl and cordierite, all of which can react with cobalt.

The difficulty with all of these diffusion or reaction processes lies in the way in which the reactants are brought together and how the reaction is controlled via temperature and time so that the polished surface of the stones is not damaged.

A first variant of the method consists in bringing the stones into close contact with metal or metal oxide powder and heating them up. This has the disadvantage that the metals or metal oxides, some of which are toxic, have to be handled. The stones must first be carefully embedded. You need suitable trays for the metal oxides, which take up a lot of expensive furnace space. The stones have to be cleaned after the treatment and the used coloring substances have to be prepared in a complex manner, taking environmental protection aspects into account, in order to be able to continue using them. Cleaning also increases the risk of stones being damaged.

The process variant of steaming or sputtering gemstones with metals or metal oxides is much cleaner. The disadvantage here is that you have a two step process. After the coating, the coated stones must be stored extremely cleanly, since even the smallest impurities can lead to burn-in before the thermal treatment.

Both processes have a common disadvantage: direct solid-state reactions often occur that do not take place uniformly over the entire surface to be colored. This also causes local damage to the polish.

To avoid these disadvantages, the invention provides that the gemstones are placed on a solid plate during the heat treatment, and the metals or metal oxides form an essential part of the plate.

A sieve plate is made from metal or metal oxide, which consists mainly of coloring materials. This sieve plate has notches in the shape of the lower part of the stones. The pavilion or pavilion and girdle disappear into the sieve plate when sieved, only the upper part peeps out. Usually a separate sieve plate is made for each stone size, but it is also possible to treat smaller stones in a plate intended for larger stones and larger stones in a plate intended for smaller stones, in particular the plate can be provided with a lid of the same composition, to completely enclose the stones.

The ground and usually also polished stones are sieved onto the sieve plates. The individual sieve plates are stacked in the oven and the treatment program started. Cobalt oxides can be used at atmospheric pressure and in an unspecified furnace atmosphere. For other oxides, such as iron oxides, you usually need a reducing atmosphere. For some materials, reduced pressure can also be beneficial. As soon as the oven has cooled down again, the stones are tipped out of the sieve plate. It is not necessary to clean the stones. Burn-in, as it occurs in the two variants mentioned above, can be avoided very well by the design options offered by a sieve plate and which are not possible with the above methods.

The avoidance of undesired solid-state reactions is first possible in the context of the invention that the contact between gemstone and plate on the circumference of the

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Steins limited by providing that the recesses in the plate are more acute than the gemstones. A better fit of the stones is achieved if the recesses in the plate that receive the faceted, pointed underside of the gemstones are conical. Here the contact between stone and slab is limited to the lower edges of the stone.

With a coloring sieve plate one has another possibility of making the diffusion or reaction between stone and coloring element more homogeneous, which one does not have with the known methods. So it is very easy to form an additional, almost arbitrarily thin diffusion layer on the sieve plate, which prevents the direct, often destructive contact between the coloring metal / metal oxide and the stone to be colored. This additional diffusion layer remains during the entire life of the plate on the surface on which the coloring element is consumed and, with increasing consumption of coloring material with the stones to be colored, only sinks deeper into the plate.

The following process can be used to produce this protective layer:

The sieve plate is not only made from the coloring oxides, but also a small proportion, usually 3 to 10%, of a non-coloring oxide. You choose one or more oxides that are also contained in the gemstone to be colored. After the sieve plate has been produced, it is conditioned. A non-coloring oxide powder, which consists of the same oxide that is contained in the sieve plate, is placed in the recesses of the sieve plate. The sieve plate is then heated over a longer period of time to a temperature slightly above the service temperature of the sieve plate. The conditioning in turn creates a thin diffusion layer that prevents a direct reaction and damage to the polish of the gemstone.

In the drawing, a cross section through a sieve plate according to the invention is shown schematically (gemstones not cut).

For procedural reasons, the plate 4 is made up of several segments 1, T, 1, which are provided with recesses 2 for receiving gemstones 3. The surface of the recesses is provided with a thin protective layer (of the order of a few μ), which prevents direct contact between the plate 4 and the stones 3.

In the illustrated embodiment of the plate 4, the gemstones 3 are only colored on the back. If the flat front part (panel) is also to be colored, the gemstones 3 can be packed in a sandwich-like manner between the illustrated and a cover-like further plate, which has recesses corresponding to the top of the gemstones 3.

Even gemstones that have a flat underside (roses) can be refined with the method according to the invention.

Then three examples of the manufacture and use of the panel shown are given:

Example 1:

Manufacture of a cobalt oxide sieve plate using a ceramic method, in which a diffusion layer of cobalt spinel is formed through conditioning and during use:

25g cobalt oxide are mixed dry with 25g gamma aluminum oxide in the planetary mill. The mixture is calcined in an aluminum oxide crucible at 100 ° C. for 6 hours. The calcined powder mixture is ground in a mortar and wet-ground with 450 g of cobalt oxide powder in a planetary mill for about 1 hour with 1.5 mm zirconium oxide grinding balls. 1.5% by weight of HoechstWax C are added to the dried powder. The mixture is again homogenized well in a porcelain mortar.

For the production of the press die, the upper part of the finished stones is ground off and the pavilion cone is glued to the lower press ram of the die at a sufficient distance. The powder is filled into the die. Approx. 1.5 to 2 g of cobalt oxide mixture is used per square centimeter of the die. The powder is pressed with 500 to 1000 bar and the sieve plate segment is ejected. The segments are debinded and sintered in one step. Small segments can be sintered according to the following program: hold at 900 ° C for 1 hour, at 1000 ° C for 6 hours, and at 1200 ° C for 12 hours.

The sintered segments are then conditioned. This is done using dry Gamma3

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Aluminum oxide is loosely pressed into the recesses of the sieve plate segments and the sieve plate is quickly heated to 900 ° C in about 1 hour. The temperature is kept for 3 hours. It is heated up to 1200 ° C in 50 ° C steps. After cooling in the oven, the excess aluminum oxide and loose spinel on the surface is brushed off. Several segments are put together to form a large plate.

On a sieve plate produced in this way, white sapphire can be colored dark blue as follows: the ground and polished white sapphires are sieved onto the plate, the plate is heated to 800 ° C. in 1 hour and the temperature is maintained for 1 hour. The mixture is heated to 1200 ° C. in 3 hours and the temperature is maintained for 30 minutes. The plates with the stones are left to cool in the oven.

Topaz is colored with a similar program. The sieve plate with topaz is heated in the oven to 800 ° C in 1 hour, the temperature is kept at 800 ° C for 1 hour. The heating continues to 1100 ° C. in 3 hours and the temperature is kept at 1100 ° C. for 1 hour.

The aluminosilicates can be treated with the same temperature program as with Topas. If you dye the kistene on the sieve plate according to the above program, the often somewhat pale natural blue coloration can be significantly improved.

Example 2:

Manufacture of a ceramic sieve plate made of cobalt oxide with a thin diffusion layer made of cobalt spinel

50 g of cobalt oxide are ground as described above and mixed with Hoechst wax. A 1: 1 calcined mixture of cobalt oxide and aluminum oxide is ground with gasoline and 5% isopropanol. The lower ram with the glued-on cones is well coated with this suspension. The layer must not be too thick, otherwise it will flake off. As above, 1.5 to 2 g of powder / cm^{1 2 3 4 5 6} filled into the die and pressed the segments. The segments are again slowly sintered and kept at 1200 ° C. for 12 hours. With the same temperature program, the coloring of a sieve plate produced in this way is stronger than that of a plate produced according to the above method. White sapphire, aluminosilicates, mullite and topaz were dyed on a sieve plate produced in this way using the above temperature programs.

Example 3:

Making a sieve plate from V₂O₅:

For sieve plates made of V₂O₅ you simply mix the powdered oxide with Hoechst wax and press the segments. The segments are sintered at 650 ° C. The still porous segments are briefly immersed in an aqueous suspension with 5 percent by weight of very fine zirconium oxide (16m²/ g) dipped and then dried. The segments are sintered again for 12 hours at 650 ° C. On this plate, cubic zirconia stones washed in dilute HCl can be colored yellow in 12 hours at 650 ° C.