Background for testing casein paint on leather

Casein paint, or milk paint, is made by mixing milk protein (usually concentrated or coagulated) with pigment and often hydrated lime, which causes a chemical reaction that makes the dried paint harder and more durable.  Roman reenactors frequently use it to paint their scuta (large legionary shields) and report that it's fairly water-resistant, more so if a wax top coat is applied.  The basic ingredients should have been available in Achaemenid Persia, and the use of milk paint is documented from the Bronze Age, so I consider it a reasonable option for period-appropriate paint.

Could it have been used on tanned leather?  I'm planning to make some more akinakes scabbards - well, sort of; I'll have to farm the wooden cores out to a professional woodcarver this time because I no longer have access to the kind of tools that are needed to produce them in a reasonable timeframe.  Anyway, wooden scabbards are greatly strengthened with a facing or other reinforcements to keep them from splitting along the seams, but aside from the chape (when it was made as a separate piece), the only reinforcements known on akinakes scabbards are facings and the only facings known are leather and embossed sheet metal.  Shaping sheet metal around the expanded throat of an Achaemenid scabbard requires great skill, so I'm going to rule it out.  That leaves leather.  As I've said before, I believe the usual leathers used in the period were fat-cured (similar to braintan and chamois) or rawhide.

Chamois is cured with fish oil, just like buff leather and German buckskin.  In the U.S. it's made from sheepskin, although the term chamois in French refers to a species of goat.  As a result, chamois is thin, elastic and soft, so it should be easy to stretch over a scabbard.  Its non-abrasive properties make it popular for polishing cars, so it can be bought in automotive and hardware stores.  I bought a three-square-foot piece from Ace Hardware for $20; I think this should be sufficient for covering two scabbards.

Milk paint is generally said to need a ground (the surface onto which it's painted) that is both hard and porous.  It works very well on raw wood.  On vegetable-tanned leather it's sometimes reported to crack.  However, as far as I know, nobody's tried it on suede leather before, so I wonder if it might not be able to soak in like thin hide glue paint.

An alternate method might be to harden the leather by soaking it in hide glue.  However, I believe this would reduce its porosity.  There are two methods of prepping a non-porous surface for being painted with milk paint that I read of:  sanding the surface to roughen it and give the paint something to "bite" into, and priming it with gesso, which is a mix of glue and chalk dust.

RAT member Feinman informs me that soaking the leather with hide glue would make it hygroscopic (meaning it would tend to absorb atmospheric humidity), which could present problems for paint adhesion.  I know of no way of dealing with this.

The other factor is whether or not to apply wax.  Over bare suede, I think that wax would tend not to seal very well.

For the time being, I intend to test everything.  I've gotten a wooden plank and attached small swatches of chamois with hide glue, each of which will then have some milk paint applied.  There are nine swatches, the three largest of which will be shared by two test treatments, so as to try every combination of leather glued only by the back, soaked with glue, sanded or not, primed with gesso or not, and waxed or not.

(The nailed-down rawhide pieces in the middle were from another project.)  As you can see, the glue-soaked chamois is brown and almost transparent.  It dries hard enough that I can barely dent it with my thumbnail, although it's not as hard as rawhide.  Also, chamois that's glued on the back but not soaked and then allowed to dry can be soaked later by rubbing more glue on the front.  The glue seems to bond the chamois pretty strongly to the wood.

Over the next few days I'll have to narrow down the possible choices of gesso and wax.  Once all the paints are thoroughly cured (allow a month) and treated, I'll test the paint's adhesion when dry, adhesion when wet, and resistance to bleeding and smearing when wet.  Stay tuned!