The international photographer (Jan-Dec 1939)

fUNdAMENTAl pHOTOqRAphic CHEIVlisTRy, PART 1 First installment of another chapter from Don Hooper's successful book on '"Basic Photography," published by Inlei national Photographer in answer to reader demand; presented in handy form for filing as reference materia Bv DON HOOPER (So enthusiastic was response to International Photographer's publication in recent issues of Chapter II of "Basic Photography" by DonHooper, which dealt with Fundamental Photographic Physics, that we are following up with the full text of Chapter III, dealing with Elementary Photographic Chemistry. Based on the famous "navy system" of photographic teaching, the book ivas compiled by Hooper to expedite his instruction courses in Los Angeles schools. The volume, published this spring, has been widely hailed as an excellent fundamental reference work. Again we remind our professional readers that while much of this material is familiar, seldom has it been gathered together in such handy form for ready reference. Readers of International Photographer who find this material worthwhile and desire to have the entire volume, may order through this publication. Only a few copies of the first edition are available. Full details will be found on Page 27 of this issue. — Ed. Note.) INTRODUCTION (29) It has been stated that photography is dependent for its present stage of development upon the science of chemistry. We can go farther than this and say that no photographic process is carried to completion without the aid of chemistry. For this reason it is essential that the Photographer have some knowledge of the chemicals he uses and the reactions they undergo in producing photographic results. This chapter will take up a short study of elementary chemistry and an outline of the various photochemical processes. The chemicals most commonly used in photography will be described and their uses given. No attempt will be made to explain the complicated nature of many chemical reactions, the idea of this chapter being to give the student a working knowledge in the use of his various photographic chemicals. ELEMENTARY CHEMISTRY All substances occurring in nature are formed from a limited number of ELEMENTS. Of these about ninety-two are known to exist. Elements may occur in the pure state or in combination in various proportions with other elements. Elements are substances which are not capable of division into any other substances than themselves. For example, silver is an element, and although we can cause silver to unite with other elements, we cannot break up pure silver into any other substance. When one element combines with one or more different elements the product of this union is called a CHEMICAL COMPOUND. As an example of this, the elements silver and chlorine can unite to form the chemical compound known as silver chloride. CHEMICAL ACTIVITY: Elements differ in their affinity for each other. A few can not be made to combine with any other element, while others, like oxygen, unite readily with others, and with many under ordinary conditions. The chemical classification of the elements and compounds, depends, to a large degree, upon their relation to oxygen. REACTIONS: The process of forming a chemical union between elements or compounds, or the breaking up of a union is usually called a CHEMICAL REACTION. The two main conditions for bringing about reactions are the presence of heat and moisture. As an example of the first, oxygen will unite with almost all elements under the influence of heat. A proper mixture of hydrogen and oxygen will not at ordinary temperatures unite to form water, but, under the influence of heat from an electric spark, they will unite explosively. As an example of the second, if we mix dry silver nitrate and sodium chloride there will be no chemical action. But by dissolving them in water they will react to form silver chloride and sodium nitrate. Practically all photographic processes employ the agency of water to bring about the necessary chemical reactions. In other words, the chemicals used are dissolved in water and the resulting solutions are capable of carrying out the chemical reactions desired. (30) CLASSES OF CHEMICAL COMPOUNDS: Chemical compounds are divided into five main classes, and for convenience these are listed below: 1. ACIDS, which are formed from non-metallic elements and which contain hydrogen replaceable by a metal. 2. BASES, which are formed from the metallic elements and when soluble are called alkalis. 3. SALTS, which are formed by the union of an acid and a base. 4. OXIDIZERS, which contain an excess of oxygen and can liberate part or all of this oxygen to other compounds. 5. REDUCERS, which have a strong affinity for oxygen and can take it from compounds containing sufficient amount of it. 8 TYPES OF CHEMICAL REACTIONS There are several types of chemical reactions which take place photographic processes. They are listed below, with examples. SIMPLE COMBINATION: This is the simplest form of chemical r action, in which two elements unite to form a compound. A very commi example of this is the rusting of iron, in which oxygen from the air unit with iron to form iron oxide. DOUBLE DECOMPOSITION: In this type of reaction two chemic compounds break up when brought into contact, usually in solution, tl various elements changing places to form other compounds. The examp already given, of silver nitrate and sodium chloride being united in sol tion, illustrates this class. Here the atoms of chlorine and nitrogen chan: places, the resultant compounds being silver chloride and sodium nitrate. COMBINATION OF AN ACID AND A BASE: When an acid and soluble base are united in solution, the metal of the base replaces the hydi gen of the acid to form a salt. As an example of this, if hydrochloric ar is added to a solution of sodium hydroxide, there will be formed sodiu chloride and water. OXIDATION: Oxygen, having a strong affinity for other elemen often unites with them in ordinary photographic processes. In additi. oxygen will unite with a number of chemical compounds used in photi raphy, which may, or may not, already contain a certain amount of ox\j:< The combination of oxygen with other elements or with chemical co. pounds is called oxidation, and is the most prevalent reaction in natu: When any substance burns it is oxidized. The rotting of wood and t rusting of iron are examples of slow oxidation. In addition to the foi{ going this term, oxidation is sometimes applied in photographic chemist to include other active elements than oxygen. Thus, the union of a active element with another element or with a chemical compound m be called oxidation. REDUCTION: This reaction applies principally to the liberation metals from their chemical combinations, and depends for its action up the presence of some substance having a stronger affinity for the remaim of the combinations than the metals have themselves. The reduction iron ore is a good example of this, while in plant life the reduction carbon dioxide by the leaves is another example of this reaction. In pi tography we have another example, in which a compound having a stro affinity for oxygen takes it away from other compounds not so greedy for From the above we see that oxidation and reduction are opposite ty] of reactions. In many cases they occur together; when one substance oxidized, some other is reduced. In some cases several of the above tyj of reactions may take place simultaneously. (311 These two types of reactions, which we have just mentioned are great value in photographic processes. Reduction is usually referred to as the act of removing oxygen from compound containing it, while oxidation usually refers to the addition oxygen to an element or compound. However, in photography these terl are used to indicate reactions in which no oxygen may be present. If compound gains one or more additional atoms during a reaction, it often said to be oxidized by reaction, and thus the substance causing t gain is called an oxidizing agent. As an example, potassium ferricyani will dissolve silver, forming silver ferricyanide and this reaction, in wlii no oxygen is involved, is called oxidation of the silver by the ferricyanii A form of reduction extremely essential to photographic processes, which no oxygen is involved, is seen in the liberation of bromine from t exposed light sensitive silver bromide grain by the action of a solution Elon, Pyro, Hydroquinone or other developing (reducing) agent. (5 developing below). OUTLINE OF CHEMICAL PHOTOGRAPHIC PROCESSES Reference has been made in previous chapters to the latent inn and the means of rendering it visible and permanent. The chemical p cesses necessary for these purposes are the main ones employed in pi tography, and before the various chemicals we use are described a br description of these steps will be given. THE PHOTOGRAPHIC "FILM": Sensitive photographic materials o sist of sheets of paper, glass or celluloid upon which the "emulsion" light sensitive salts usually suspended in gelatine is spread. This gelati emulsion clings firmly to the support, and is closely packed with grains the sensitive salt. In the case of negatives, the support is transparent, wli for positives, or prints, it is usually on paper and the image is viewed