Patent GB-235210-A (GB)
Title: Improvements in or relating to the manufacture of photographic emulsions
IPC: G03C1/09
Filed: 1925-06-05
Abstract: A sensitizing material is extracted in solution from organic tissues (e.g. hides, bones, cereal grains and leguminous plant seeds) and has the property of enhancing the sensitivity and other properties of photographic emulsions of various kinds.  The material is contained in useful quantities in the sterol-containing extracts or fractions of animal and plant extracts.  Sources of the material are bye-products from the treatment of hide, bones and gelatine manufacture, such as deliming solutions and bye-products from cereal preparations. Sources are of greater utility which have not been submitted to excessive oxidizing action which tends to destroy the active property of the material.  As one example of preparation from animal sources, limed hide stock is agitated in a 1 per cent solution of hydrochloric acid, until a thick piece when cut across shows slight acidity to litmus at the inner parts of the cut. The deliming solution is then drawn off, concentrated at reduced pressure, and the concentrate is boiled until a coagulum or p  precipitate is formed, which carries with it the sensitizing material, and is separated as by decantation, filtering or centrifuging off.  It can then be washed.  The coagulum is then preferably dried and ground.  The sensitizing material is then extracted by a suitable solvent (e.g. alcohol) as by an extractor working on the Soxhlet principle of the continuous type.  The alcoholic extract contains sterols and strong sensitizing materials as also does the dry residue obtained by evaporating it. By another method from a deliming solution obtained as above, an inorganic precipitate is formed to which the sensitizing material adheres.  Potash alum is added to the deliming solution, the liquid brought nearly to boiling point, and caustic soda added to give a precipitate of aluminium hydroxide, addition being made until the pH value is 8 as shown by phenol red.  The precipitate is allowed to settle, separated, dried and ground. Sensitizing material is extracted by ethyl alcohol, prolonged storage being avoided.  Instead of using alcohols as the extracting solvent, which dissolves also certain impurities, ligroin or a volatile petroleum, e.g. petroleum ether, may be used.  Since alcoholic extracts enter emulsions better however, the ligroin extracts are preferably evaporated to dryness and the residue taken up with alcohol.  In an example of preparation from vegetable sources, lima beans are ground to a coarse flour, and, avoiding axidization, the flour is extracted with ligroin boiling between 40 DEG  to 60 DEG  C.  The extract is evaporated and the residue taken up in alcohol.  Alternatively again mustard seed is ground in water, allowed to ferment for say 12 hours at room temperature and extraction effected by shaking with ligroin.  The ligroin extract although a strong sensitizing material can be further concentrated and purified.  To remove any saponifiable oils, caustic soda is added to an alcoholic extract, the mixture boiled in apparatus having a reflux condenser until the oils are saponified. The solution is then evaporated to dryness and the residue taken up in sufficient water.  Ligroin, or related petroleum ether, is added with shaking, the sensitizing material entering the ligroin whilst the oils and any glycerine liberated stay in the water.  The ligroin extract is then separated out, the ligroin evaporated, and the residue taken up in alcohol.  The sterol may be removed from such extracts, wholly or partially, by crystallization, by evaporating to dryness, taking up the dried residue in just sufficient warm, but not boiling, ethyl alcohol (say at 70 DEG  to 80 DEG  C.) to dissolve it, cooling until sterol crystals form, removing the crystals, washing them with a little ligroin, adding this wash to the liquid from which the crystals came, and finally concentrating by evaporation the combined liquids preferably at less than atmospheric pressure.  The sensitizing material appears not to be truly soluble in water but to form a colloidal solution therein, but the solutions in alcohol and ligroin are true ones.  It is stable if excessive exposure to oxidizing influences is avoided.