Photographs have a base material coated with a light sensitive emulsion.
Daguerreotypes use a copper sheet coated with silver. They are displayed behind glass covers to protect the polished silver surface with a tape border bind and placed in hinged cases. Calotypes use a base of silver iodide and gallo-silver nitrate as emulsion. Albumen prints used paper as base and egg whites and salt as emulsion. Ambrotypes use a base of glass (tintypes use blacken iron sheet) with a syrupy liquid (collodion) as emulsion. Modern prints have a paper base and gelatin emulsion.
Determining factors for longevity include the film base type, proper processing, and proper storage. Factors that contribute to the deterioration of photographs include: light, temperature and humidity, fluctuations, pests, mold/mildew, improper handling, air pollutants, water and fire damage.
- Daguerreotypes are physically fragile, and the silver tarnishes easily.
- Calotypes last a long time depending on the rag content of the paper used.
- Albumen prints are affected by light, chemical air contamination, and the album pages used to protect the images.
- Ambrotypes crack and yellow due to its chemical makeup.
- Silver halide emulsions (modern prints) last for archival storage, but are also subject to environmental contamination.
Early film coated with nitrate base material is prone to combustion if stored in uncontrolled temperatures. Nitrate film use was replaced with acetate-base films which last longer.
Cellulose acetate films were later discovered to degrade emitting acids (also known as vinegar syndrome).
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