Can someone explain me the chemical reactions in printing? I mean what's it on the paper surface, what's the reaction in the liquids etc. If you can help me i'll be really happy

-- Benan Ilhanli (benan@superonline.com), February 28, 1999

Answers

It's a complex subject, and I'm no chemist. You'll find details in books such as the Focal Encyclopedia of Photography.

-- Alan Gibson (Alan.Gibson@technologist.com), March 01, 1999.

I found this info that might help you on a website but I lost the address, here's what I found anways: Light exposes tiny crystals of AgBr (silver bromide) that are suspended in a thin layer of gelatin coating on the paper surface. When these crystals are exposed to light, several free silver atoms form, as light passes through the lighter parts of the negative the highest concentrations of the Ag form, and the longer the exposure the more Ag particles form. The latent image is not visible to the eye, until More AgBr is converted to silver metal with the developer. The AgBr grains react with the developer to produce more black silver metal- the image begins to appear. The stop bath simply rinses the AgBr off the paper to stop the developing process (for this reason, a real stop is unnecessary; water does a fine job). The fixer, sodium thiosulfate reacts with the remaining AgBr to form a water soluble salt that can be washed out in the final step.

-- Lara Rafton (dweege@goplay.com), April 26, 1999.

The previous post is almost correct. This explanation is extremely simplified.

The emulsion is made of gelatin with silver halide. Halogens are elements like bromine, chlorine, and iodine. Silver nitrate (silver dissolved in nitric acid) is slowly dripped into a gelatin and halide mixture. The silver nitrate combines with the halogen in the gelatin to form the bromide, chloride, chloro-bromide, or bromo-chloride silver halide crystals in the emulsion. The emulsion is then coated on the support like paper for photographic paper, or acetate or estar for film (plastics).

When photons of light strike the silver halide crystals in the emulsion, they cause a photo-chemical reaction in which the electrical charge of the silver halide crystal is changed as the photon (and its energy)is absorbed by the silver halide crystal. The developer molecules (which also have an electrical charge) are attracted to the silver halide crystals because of their opposite electrical charge, and oxidize the silver halide crystal, changing it from a halide compound to silver metal.

Developers are alkaline (PH more than 7.0), and when the print (or negative) is put into a stop bath that is acid (PH less than 7.0), the development is stopped because the developer PH has been changed. Water is not a good substitute for a stop bath because it only rinses the developer off the top of the emulsion, and the developer still in the emulsion continues working and making oxidation by-products. The developer, exhausted developer, and development by-products which are carried over into the fixer will cause a change in the fixer from acid to neutral or alkaline. When this happens, staining or formation of dichroic fog will result. Also, this will greatly shorten the life of the fixer. Always use an acid stop bath.

Finally, the fixer acts on the unexposed and undeveloped silver halide crystals combining with them into different chemical compounds that do not react with light. Some of the compounds are retained in the fixer, and some remain in the emulsion which is why you need to wash the prints or film thoroughly and use a chemical washing aid (hypo eliminator or hypo clearing agent).

If you really want to know more - contact me by e-mail and I will give you the titles of two books on photo chemistry.

-- steve (swines@egginc.com), May 03, 1999.