Glycerin

Glycerin (glycerol; C₃H₈O₃; CAS 56-81-5) is a humectant (moisture-retaining compound) and viscosity modifier with a handful of specialty photographic uses. Non-toxic, food-grade, and water-miscible, it stays in a coating or solution long after the water has dried, keeping gelatin and gum layers flexible.^[1]

Photographic uses

• Gum bichromate printing: 1–5 % glycerin in the gum-pigment-dichromate coating slows drying, extends the working window, and controls print contrast. Higher glycerin lengthens the exposure-development tonal scale; lower concentrations give harder-edged prints.
• Emulsion flexibility aids: Added to some hand-coated silver-gelatin emulsions and carbon-tissue sensitizers to prevent cracking as the gelatin dries.
• Wet plate collodion preservative coatings: Glycerin / gum-arabic mixtures keep prepared plates workable longer than the classical "plate must be exposed while wet" rule would suggest.
• Inkjet digital negative substrate coatings: A humectant component in some hand-mixed digital-negative inkjet coatings for alternative printing.

Practical notes

Supplied as clear, odorless, very viscous syrup. Food-grade (USP) and technical grades are both suitable for photographic use; the USP grade is readily available at pharmacies. Highly hygroscopic — containers should be kept tightly closed to prevent water absorption that would change working concentration.

Related compounds

Ethanol and isopropyl alcohol are drying solvents with the opposite effect — they accelerate rather than retard drying.