Digital BW, The Print

Gelatin is an amazing polymer with fascinating properties that enable traditional photographic processing.  It's a hard dry polymer with protective barrier properties when it's below its glass transition temperature (Tg), but it moves to a highly permeable gel state when above Tg. The Tg of most polymers is fairly independent of humidity, but not so with gelatin. This relatively unique property of moisture content influence on the Tg is a "two-edged sword". It makes conventional film and photo paper processing possible, but it creates a long term environmental weakness for the image.

We take it for granted that gelatin goes into a gel state and becomes highly permeable when it gets immersed in water.  Gelatin is above Tg when wet. However, gelatin actually reaches Tg and switches to the gel state when the humidity at typical room temperature (approx. 21C/70F) reaches about 75%RH (a common real-word event in many areas of the world). Ever seen a traditional silver-gelatin print with silver tarnish or stuck to glass in a frame? That's a common outcome of a traditional photographic print traversing the glass transition at typical room temperatures due to prolonged high humidity events.  We need gelatin's Tg properties to carry out the chemical photographic process, but I'd probably choose something else if I'm looking for long term protection of inkjet colorants underneath a post coating treatment. 

Gelatin also has some fascinating anisotropic physical properties that chemical hardeners (e.g. formaldehyde) can significantly influence, but I'll save that discussion for another day. 

Acrylic polymers like Premier Print Shield have a pretty good track record and are widely used to protect paintings these days (especially paintings done with acrylic paints).  Research at both Wilhelm Imaging Research and Aardenburg does suggest for the most part that acrylic coatings cut down on photo-oxidation of pigments so improve lightfastness, gas fastness, etc.  However, the costs, the need for good ventilation during application, and application challenges of spray or roller coating without defects means there is no free lunch in the quest for durable, abrasion resistant coatings.

cheers,
Mark
http://www.aardenburg-imaging.com