hardener

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One that hardens, especially a substance added to varnish or paint to give it a harder surface or finish.

An ingredient (potassium alum) of the photographic and radiographic fixing solution that serves to harden the gelatin of the film to prevent softening and swelling of the gelatin.

1. A chemical (including certain fluosilicates or sodium silicate) applied to concrete floors to reduce wear and dusting.
2. A material added to a paint or varnish vehicle to increase the gum or resin content, or to increase rate of oxidation, so as to cause an increase in hardness of the drying film.
3. The chemical component in a two-component coating or adhesive which causes the resin component to harden.

In solutions at temperatures above about 25 °C gelatin begins to absorb excessive amounts of liquid and to soften; an unhardened photographic emulsion may become reticulated or even detached from its substrate. To avoid this, most emulsions are treated with a hardening process during manufacture. Hardening solutions such as formanal (formaldehyde), in alkaline solution, crosslink the collagen molecules and limit the swelling. For high-temperature development (35° plus) a further similar prehardening bath is desirable, as well as the addition to the developer of sodium sulphate. Trivalent ions such as aluminium (Al³⁺) also have a powerful hardening effect, and are added to most negative fixing baths in the form of potassium alum. Chrome alum is even more effective, but is used as a separate final bath, as it is unstable in acid solution.

— Graham Saxby

A substance used to harden the gelatin of the emulsion on an x-ray film.