hide glue

Gelatin Molds For Plaster Casts II – Ye Olde (and new) Apothacary-ness

Developing, or more precisely re-developing, technology that was once common practice requires lots of mental noodling sprouting from the question, “Really, how did they do that?”  In the end it all comes down the the world we artisans inhabit, the world of Applied Materials Science.  Fortunately for me the base material, hide glue, was plentiful in the shop so I had plenty of raw material to work with.  In the pursuit of gelatin molds for cast ornamental plaster my proof-of-concept work revolved around the observations of how molten hot hide glue (the “gelatin”) actually cures into a rigid adhesive layer and the changes in physical properties while en route.

As cooked glue goes from hot liquid to hardened mass the first step is the one of greatest importance for this undertaking.  At a particular point in the process — exactly where and when depends on a number of factors including the concentration of glue solids dissolved in the water, the grade of glue, the ambient temperature, relative humidity, barometric pressure, and probably phase of the moon — the molten glue mass forms a semi-solid gelatin, which is the stage I am interested in both exploiting and maintaining in stasis.  As long as the mass stays flexible it can be used and re-used as a molding material.

A  major concern for this practice is that the gelled/flexible mass be water resistant.  Though that seems, and is, obvious the accomplishment of that feature requires a bit of forethought.  If casting plaster is the end goal, and for me and the ancients it was, the mold for holding the plaster as it was poured and cured had better be able to withstand the incursion of the copious quantities of water involved.

A second consideration is that the gelled mass, once transformed from an amorphous blob into a detailed mold, be tough enough to impart said details and allow for the set plaster casting to be de-molded without destroying the mold itself.

And finally, in order to be used repeatedly the gelatin mold must be preserved over some indefinite period of time.

Reflecting back on protein chemistry and historical craft/art practice I conclude that the ancients accomplished all three of the previous items more-or-less in one step: they added the most potent protein crosslinker/preservative they could find, formaldehyde.  It has long been understood that exposing or incorporating formaldehyde into collagen matrices renders them water-resistant, or “hardened.”  Think embalming.  Think taxidermy.  Think of the ancient practice of exposing bowstrings and wrapping to the smoke of a wood fire, from which formaldehyde is a by-product.

Since formaldehyde is so noxious I do not have it in my chemical inventory and instead relied on another chemical from the world of film-based photography that performs an analogous function of crosslinking or “hardening” gelatin films.  In practice I found this to be an admirable option for a couple reasons, and not so good in another,  Not problematic per se, but requiring another consideration.

To be sure the photographic gelatin hardener performs admirably in imparting water-resistance and integral toughness to a gelled mass.  I observed it also extends the timeline for the gelled state considerably, apparently retarding the water egress that turns gelled collagen into a hard, glass-like film.   This was perhaps my (and their) first ace in the hole.  A second observation beneficial to the process was that the plaster itself, being integrated into and reacted by water, served to moisten and thus keep the mold flexible   Every time plaster was cast into the mold, it re-plasticized the mold mass.

However, the current safer chemistry of the gelatin hardener does not impart the same biocide/preservative effect that was accomplished previously by formaldehyde.  Thus my initial mold attempt turned into a rotting mass of oozing slime in a few days.  Not an un-solvable problem, but a stinky, sticky mess.

Back to the drawing board.  Sorta.

Next time – Ye Whole Sheebang.  This time with lots of pctures.