Hide glue paint tests - gearing up

Thesis
It is possible to paint soft-tanned leather goods using basically period materials.

Background
Animal glue is one of the oldest-known adhesives in the world, second only to plant resins.  Archaeological evidence dates back to the Neolithic, written references back to the Bronze Age.  It's made by cooking down animal tissues that are high in collagen, a connective protein; these tissues are typically bone, tendon, or particularly skin (strictly, hide glue is only that made from skin).  Today it's still widely used by woodworkers, especially those engaged in making and repairing stringed instruments.

Animal glue is also used as a paint sizing, the substance in which the pigments are dissolved and by which they adhere to the canvas.  Plains Indians used a thin solution of glue for painting buckskins.  It's supposed to soak partly into the hide, remaining semi-flexible - the rough, "open" finish on brain-tan probably helps with the absorption.  Since the most common leathers of the ancient Mediterranean were similar to brain-tan, this sounds like a plausible solution for us.

Standard "hot" hide glue must be held at around 140 degrees Fahrenheit while working:  colder and it will gel, much hotter and the collagen will begin to break down, weakening it.  However, hide glue can be kept liquid at room temperature by the addition of an anti-gelling agent, commonly salt or urea.  While I can't prove that it was used historically, urea was always readily at hand in the form of urine.  That may sound disgusting, but when you think about some of the things people had to do to make leather, adding pee to glue suddenly doesn't sound that farfetched.  Thankfully, we don't have to trouble ourselves with that, because premixed liquid hide glue is already available at some hardware stores.

While liquid hide glue has a lesser reputation than hot hide glue due to the urea weakening its bonding power, and unused portions will break down over time, Stephen Shepherd of Full Chisel has argued that fresh liquid hide glue is still one of the stronger glues available.  In any case I'm not sure whether the strength of the glue is as critical in painting as it would be in woodworking.

There is one other consideration:  While Greece is pretty dry in the summer, most of our stuff would have been exposed to rain sooner or later in ancient times.  Hide glue isn't waterproof by itself, but can be made highly water-resistant by the addition of a small amount of alum.  Again I have no reason to think this was done, but alum was used as a mordant and leather tannage, so it was at least available.  The containers and brushes used with glue that has alum added, must be washed thoroughly before the paint dries or it will be difficult or impossible to wash out afterward.

Materials and methods

Liquid hide glue
Water for diluting glue
Alum (aluminum sulfate)
Earth pigment
Suede leather scraps

Alum is recommended in amounts of 1-2 percent.  Too much can cause the glue to gel prematurely.  Assuming it has a similar weight to salt, for one ounce of glue (before dilution), I estimate one twentieth of a teaspoon would be the correct amount, but since it's so hard to measure in such quantities, I'll try it by eye - slightly less than half of an eighth of a teaspoon ought to be close.

In order to mix the alum thoroughly into the glue, it should probably be dissolved in the water first.  An exact ratio of water to glue is not given in the Crazy Crow instructional, so I'll begin with a 1:2 ratio which hopefully won't make it too dilute to begin with.  Let's start with a tablespoon of glue, a teaspoon and a half of water, and half of a half of an eighth of a teaspoon of alum.

The bottle on the left is a modern acrylic leather paint I have on hand, which I'll be comparing to the hide paint to see whether it's acceptable as a more convenient substitute.

Questions
Does hide paint remain flexible enough when dry for use on soft leather goods (gorytoi, bags, clothing)?
Does waterproofing with alum affect the paint's flexibility?
Does acrylic leather paint provide a similar appearance and degree of flexibility and water-resistance to alum-treated and alum-free hide paints when used on the same leather?