Claims
- 1. A method of preparing a silver halide emulsion of the twinned type which comprises the steps of (a) forming in a colloid dispersing medium silver halide crystals containing at least 90 mole % iodide, these said crystals being predominantly of the hexagonal lattice structure and then chemically sensitising the said silver halide crystals which contain at least 90 mole % iodide, (b) mixing in the dispersing medium containing the said silver halide crystals an aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium bromide or chloride or mixtures thereof so forming twinned silver halide crystals containing iodide and the halide or halides being added, (c) adding a silver halide solvent to the dispersing medium and so causing the growth of the twinned crystals by Ostwald ripening, optionally (d) causing the twinned crystals to increase in size by adding to the colloidal dispersion further aqueous silver salt solution and further alkali metal or ammonium halide and then finally optionally (e) removing the water-soluble salts formed and chemically sensitising the surface of the silver halide crystals of the emulsion.
- 2. A method according to claim 1 wherein the chemical sensitisation in step (a) is gold or another noble metal sensitisation.
- 3. A method according to claim 2 wherein the other noble metal is platinum, iridium or rhodium.
- 4. A method according to claim 1 wherein the chemical sensitisation in step (a) is effected by a compound of a heavy metal.
- 5. A method according to claim 4 wherein the heavy metal compound is a salt of lead or bismuth.
- 6. A method according to claim 1 wherein the chemical sensitisation in step (a) is sulphur or selenium sensitisation.
- 7. A method according to claim 1 wherein the chemical sensitisation in step (a) is reduction sensitisation.
- 8. A method according to claim 7 wherein the reducing agent used is a stannous salt, thiourea, a hydrazine or formaldehyde.
- 9. A method according to claim 1 wherein two types of chemical sensitisation are carried out in step (a).
- 10. A method according to claim 9 wherein the two types of chemical sensitisation are gold and sulphur sensitisation.
- 11. A method according to either claim 2 or claim 6 wherein the sensitisation is carried out at a fixed pAg between 8 and 9 and at a fixed pH between 5 and 7 at an elevated temperature between 50.degree. C. and 60.degree. C.
- 12. A method according to claim 5 wherein the sensitisation is carried out at a pH of approximately 3.
- 13. A method according to claim 1 wherein step (d) is carried out and the silver halide crystals formed in step (b) constitute the core of final silver halide crystals and the halide added in step (d) constitutes the shell of the final silver halide crystals.
- 14. A method according to claim 1 where in step (b) the halide precipitated during at least the first part of the step is predominantly chloride and in step (d) the halide precipitated at least during the final part of the step is predominantly bromide.
- 15. A method according to claim 1 wherein the surface of the silver halide crystals are chemically sensitised in step (e).
- 16. A method according to claim 15 wherein the chemical sensitisation is sulphur and gold sensitisation.
- 17. A method according to claim 1 wherein the surface of the silver halide crystals is fogged in step (e).
- 18. A method according to claim 17 wherein the fogging is carried out by use of a reducing agent together with a compound of a metal more electro-positive than silver.
- 19. A method according to claim 18 wherein an electron-trapping compound is adsorbed on the surface of the silver halide crystals.
- 20. A silver halide emulsion when prepared according to claim 1.
- 21. Photographic material which contains at least one layer which contains a silver halide emulsion according to claim 20.
Priority Claims (2)
Number |
Date |
Country |
Kind |
24001/76 |
Jun 1976 |
GBX |
|
22728/78 |
May 1978 |
GBX |
|
CROSS-REFERENCE TO RELATED PATENT APPLICATION
This application is a continuation-in-part of U.S. application Ser. No. 799,040, filed May 20, 1977, now U.S. Pat. No. 4,150,994.
US Referenced Citations (11)
Foreign Referenced Citations (2)
Number |
Date |
Country |
1335925 |
Oct 1973 |
GBX |
1469480 |
Apr 1977 |
GBX |
Continuation in Parts (1)
|
Number |
Date |
Country |
Parent |
799040 |
May 1977 |
|