Title:
Synthetic star rubies and star sapphires, and process for producing same
United States Patent 2488507


Abstract:
This invention relates to a process for developing asterism in natural and synthetic corundum crystals, such as crystals of ruby and sapphire, which contain oxide of titanium dissolved in alumina. The invention is also concerned with asteriated single crystals of synthetic corundum as articles...



Inventors:
Burdick, John N.
Glenn Jr., John W.
Application Number:
US77094247A
Publication Date:
11/15/1949
Filing Date:
08/27/1947
Assignee:
LINDE AIR PROD CO
Primary Class:
Other Classes:
501/86, D11/89
International Classes:
C30B29/26
View Patent Images:
US Patent References:
1814219Process of improving certain physical properties of synthetic precious stones1931-07-14
1161620N/A1915-11-23
1004505N/A1911-09-26



Description:

This invention relates to a process for developing asterism in natural and synthetic corundum crystals, such as crystals of ruby and sapphire, which contain oxide of titanium dissolved in alumina. The invention is also concerned with asteriated single crystals of synthetic corundum as articles of manufacture, for example synthetic star rubies and star sapphires.

For more than thirty years massive nongranular synthetic rubies and sapphires of gem quality have been produced commercially on a large scale by the process of Verneuil, as disclosed in U. S. Patents 988,230 and 1,004,505. In that process powdered alumina, with or without small percentages of coloring oxides, drops through an oxy-hydrogen flame, fuses, and accumulates on a refractory support until a long, narrow, approximately cylindrical boule of the desired size is formed. Boules weighing several hundred carats are common. For rubies the color may be imparted by chromium oxide. In blue sapphires, small quantities of titania and iron oxide are often used. Sapphires of other colors can be produced by including with the alumina small quantities of one or more other oxides, such as those of manganese, cobalt, vanadium, and nickel.

There are occasionally found in nature asteriated rubies and sapphires which, when cut en cabochon, i. e. with a convex surface opposite a substantially plane surface, exhibit a welldefined six-rayed star when observed under reflected light, particularly from a point source.

Such crystals, usually called star rubies or star sapphires, are highly prized, and there is a great demand for them. Also prized are rubies and sapphires having such a structure and cut in such a manner as to show but one ray.

Although the Verneuil process for manufacturing synthetic corundum crystals, and the resulting product, have been subjected to intensive research for many years, asteriated rubies and sapphires were never produced synthetically prior to the present invention. In fact, according to Kraus and Slawson, in their book "Gems and 4 Gem Materials," (1941) "As it has not been possible to produce synthetic rubies or sapphires which show asterism, star rubies and star sapphires must have been cut from the mineral." The principal object of the present invention is to provide asteriated single crystals of synthetic corundum, such as synthetic ruby and sapphire of various colors. Another object is to provide cut synthetic star rubies and star sapphires. 6 Other objects are to provide a process for developing asterism in single crystals of ruby and sapphire containing oxide of titanium; to provide a process for obtaining star ruby and star sapphire gemstones from non-asterlated single crystals of ruby and sapphire containing oxide of titanium; and to provide a process for intensifying asterism in asteriated ruby and sapphire crystals.

The term "massive non-granular," as used herein, is intended to designate single crystals as distinguished from a sintered or agglomerated mass of granules. Moreover, this term is further intended to designate crystals which are larger than such tiny particles or granules of corundum as are commonly employed in abrasives, those crystals designated by the term "massive nongranular" being, indeed, large enough to be used in jewelry settings and of a size sufficient that any asteriation present in them can be readily apprehended and appreciated with the naked eye.

The term "of gem quality," as used herein, is intended to mean that the crystal is sufficiently perfect to warrant cutting, polishing, and offering for sale as an ornament.

We have found that asterism can be developed artificially in a massive nongranular single crystal of ruby or sapphire of gem quality which contains oxide of titanium dissolved in alumina. This is 3 accomplished by heating such a crystal at a temperature within the range between 1100* C. and 1500" C., and maintaining the crystal constantly at a temperature within that range until a compound of titanium (probably titanium dioxide) precipitates along prominent crystallographic planes of the crystal. The length of the heating period required varies as an inverse function of the temperature, being less at the higher temperatures than at the lower temperatures. The 4o best heating time for any selected temperature is readily determinable by trial. As examples of suitable heating schedules, asterism was successfully produced in both synthetic ruby and synthetic blue sapphire boules after 72 hours at ,5 1100' C., after 24 hours at 1300* C., and after 2 hours at 1500° C. Below 1100° C. no asterism developed. Above 1500" C. no precipitation of the oxide of titanium occurred.

For best results, the alumina powder from o which the crystals are grown should contain at least 0.1% and not more than 0.3% of TiO2.

Above 0.3% TiO2, considerable difficulty is encountered in growing the boule.

Among the ruby and sapphire crystals suc5 cessfully asteriated by the above-described proeedure were a white sapphire half-boule grown from a powder containing 99.9% of Alab and 0.1% of TiO0; a blue sapphire half-boule grown from powder containing 99.4% of AlsOs, 0.1% of TIO2, and 0.5% of PesO3; and two ruby halfboules grown from powder containing 2.0% of CrsO,; 0.1% and 0.2%, respectively, of TIOs, balance Al03.

Synthetic corundum whole boules treated by our novel process are characterized by having a. generally cylindrical peripheral surface, and a cloudy, opalescent skin which contains a heavy concentration of the precipitate of titanium compound, probably titanium oxide. The inner portion of the boule is substantially free from precipitated titanium oxide. Of course, when a whole boule is split along its longitudinal axis in a conventional manner to form two half-boules, each segment has such a skin only on its cylindrical surface, and is substantially free from precipitated titanium oxide throughout the rest of its mas After heat-treating the crystal as described above, a gemstone is cut en cabochon in such a way that the base of the gemstone is normal to the C-axis with the C-axis extending symmetrically follow through the center of the stoneandthrough the center of its convex crown. The skin may be on either the base or the crown. A well-defined six-rayed star centered in the crown of the stone is obtained by this manner of cutting. If desired, the asteriated gemstone can be cut so that less than six rays show, as by cutting the stone with the C-axis at an angle to, rather than coincident with, the normal to the plane of the base of the stone.

After cutting an asterlated ruby or sapphire en cabochon it is frequently desirable to heat or reheat it in the range from 1100° C. to 1500' C. to intensify the visibility of the star.

The synthetic asteriated crystals, both cut and uncut, contain curved growth lines and microscopic or submicroscopic oriented acicular crystals which are probably rutile (TIOs). The visible precipitate is distributed primarily in the surface layer or skin of the stone. Natural sapphires and rubles usually have straight rather than curved growth lines; and their rutile crystals (or other oriented precipitate) are distributed throughout the crystal (although especially concentrated in particular growth zones defined geometrically by the crystal faces) and are usually non-uniform in size. Furthermore, the synthetic crystals often exhibit curved striae in the cloudy precipitate zone, apparently corresponding to growth lines of the boule.

The refraction and interference-diffraction of reflected light from the oriented, needle-like precipitate crystals is believed responsible for the star effect We claim: 1. A process for developing asterism in massive nongranular single crystals of ruby and sapphire containing oxide of titanium in solution, comprisIng heating such a crystal at a temperature with in the range between 1100* C. and 1500' C, and maintaining said crystal at a temperature within said range until titanium oxide precipitates in said crystal.

2. A process for developing asterism in massive nongranular single crystals of ruby and sapphire containing oxide of titanium in solution, comprising maintaining such a crystal at a temperature constantly between 1100° C. and 15000 C. for more than 2 hours, the length of the heating period varying as an Inverse function of the temperatur.

3. A process for developing asterism in massive nongranular single crystals of ruby and sapphire containing oxide of titanium in solution, comprising percipitating oxide of titanium in such a crystal by maintaining such a crystal at a temperature constantly between 1100* C. and 1500° C. for between 2 and 72 hours, the length of the heating period varying as an inverse function of .10 the temperature.

4. A process for developing asterism in massive nongranular single crystals of ruby and sapphire containing oxide of titanium in solution, comprising maintaining such a crystal at a temperature of about 1300° C. until oxide of titanium precipitates in said crystal.

5. A process for developing asterism in massive nongranular single crystals of ruby and sapphire grown synthetically by Verneuil's procedure from an alumina powder containing as an essential ingredient 0.1% to 0.3% of T102, said process comprising heating such a crystal at a temperature within the range between 1100° C. and 1500° C., and maintaining said crystal at a temperature within said range until titanium oxide precipitates out of solution.

6. A process for preparing star rubies and star sapphires from non-asteriated massive nongranular single crystals of ruby and sapphire containing oxide of titanium in solution, said process comprising heating such a crystal at a temperature within the range between 1100° C. and 1500° C.; maintaining said crystal at a temperature within said range until titanium oxide precipitates in said crystal; and thereafter cutting a gemstone from said crystal en cabochon in such a way that the base plane of said gemstone is normal to the C-axis of said crystal.

7. A process as claimed in claim 6, also comprising reheating said cut gemstone and maintaining the temperature thereof within the range between 1100° C. and 1500* C. to intensify asterism therein.

8. An asteriated massive nongranular single crystal of synthetic corundum of gem quality containing a precipitate of a compound of titanium.

9. An asteriated massive nongranular single crystal of synthetic ruby of gem quality containing a precipitate of a compound of titanium.

S0 10. An asteriated massive nongranular single crystal of synthetic blue sapphire of gem quality containing a precipitate of a compound of titanium.

11. An asteriated massive nongranular single Scrystal of synthetic corundum of gem quality containing a precipitate of oxide of titanium, and characterized by having curved growth lines internally thereof.

12. An asterlated massive nongranular synS0 thetic corundum boule segment of gem quality having a generally cylindrical surface, said boule segment being substantially free from precipitated titanium oxide throughout the center portion of its mass but having a skin including and * coextensive with said cylindrical surface containing a precipitate of titanium oxide.

13. An asteriated massive nongranular synthetic corundum whole boule of gem quality having a generally cylindrical peripheral surface, said boule having a cloudy opalescent skin including and co-extensive with said peripheral surface, said skin containing a precipitate of titanium dioxide, the center portion of the man of said boule being free from precipitated yI titanium oxide.

14. An asteriated massive nongranular synthetic corundum gemstone cut en cabochon with its base normal to its C-axis, said gemstone being substantially free from precipitated titanium oxide throughout part of its mass but having a skin containing a precipitate of titanium oxide; said gemstone having curved growth lines.

15. A gemstone as claimed in claim 14, wherein said skin includes the crown of said gemstone.

16. A process which comprises providing a massive nongranular single crystal of synthetic corundum of gem quality containing oxide of titanium in solution; and precipitating oxide of titanium in said crystal by maintaining said crystal at a temperature between 1100* C. and 15000 C. for more than 2 hours to develop asterism therein.

17. A process of producing an asteriated crystal of the group consisting of ruby and sapphire which comprises growing such crystal by Verneull's procedure from a powder consisting principally of alumina and containing oxide of titanium as an essential ingredient; heating the crystal so grown at a temperature between 1100'C. and 1500° C.; and continuing such heating of said crystal at such temperature for more than two hours until a cloudy precipitate forms along crystallographic planes in said crystal to develop asterism therein.

18. A process for developing asterism in single crystals of ruby and sapphire grown synthetically from an alumina powder containing oxide of titanium as an essential ingredient, said process comprising heating such a crystal at a temperature between 1100' C. and 1500° C. for more than two hours, the length of the heating period varying as the inverse function of the temperature. 19. A crystal resembling, both in appearance and asterism, a natural star sapphire or ruby but composed of a synthesized mass of corundum and pigmenting material with included needle-like crystallites of a precipitated titanium compound oriented therein, the synthetic character of the crystal being detectable by curved growth lines which appear internally thereof as contrasted with the straight form of any growth lines which appear in the natural mineral crystals.

JOHN N. BURDICK.

JOHN W. GLENN, Ja.

REFERENCES CITED 20 The following references are of record in the file of this patent: UNITED STATES PATENTS Number Name Date 25 1,004,505 Verneuil --_.-------Sept. 25, 1911 1,161,620 Coulter -------___ Nov. 23, 1915 1,814,219 Jaeger et al. -----_- July 14, 1931 OTHER REFERENCES 1. Gems and Gem Materials, Kraus and Shawson.

2. The Art of Gem Cutting, Dake and Pearl.

3. Introduction Gemology, Webster & Hinton.