Testing casein paint on leather, part III

Dammit.

Our test board cured for an entire month.  Last week I added some thin layers of carnauba wax paste, which is, admittedly, not something that would've been available at the time (carnauba palms are native to Brazil), but I was attracted to the fact that it's harder than beeswax and has a higher melting point.  It's difficult to see, but on some of the swatches it dried to a whitish waxy appearance.  At this point, my favorite was the glue-soaked, sanded leather with a wax topcoat and no gesso (lower right corner), which was smooth and a bit glossy.

I wanted to first test the paint against rough surfaces and sharp implements to see if it would flake.  It didn't, though the knife and half brick scuffed the paint applied over unglued leather badly.  The glued/sanded/waxed/un-gessoed swatch looked to be the most durable, as both implements glided over it leaving superficial scratches.

The real test, of course, would be the rain.  I waited until today when it was convenient to set the board outside for an hour, as I figure these are realistic conditions.

In the meantime, I cut up a whole bunch of undyed, semi-bleached linen swatches, as I figure these would be the most likely to show paint smudges if someone's clothing were to come into contact with wet paint.  When the board came inside, I rubbed one swatch of linen against each swatch of paint.

At this point, it became difficult to read the labels, but that didn't matter.  NONE of the paint swatches adhered well enough for practical use; each one smudged badly onto the linen.

Sadly, the glue-soaked but un-gessoed swatches fared by far the worst, as the glue absorbed water and turned liquid.  The paint slid right off of them.  The gessoed ones didn't do much better.  The wax topcoats gave no noticeable protection.  In fact, the leather swatch with no treatment at all (corresponding to the linen at the bottom of the leftmost column) did best - or, at least, least worst - although as you can see, it still smudged.

So where do we go from here?
The thing is that milk paint is normally applied to raw wood, and it's said to be durable in this application, particularly if coated with a drying oil.  This leaves us with several options:
1)  Bare wood scabbards with only a drying oil or wax finish.
2)  Painted wood, again probably having a water-resistant finish.
3)  Unpainted leather facings, perhaps vat-dyed in a single color (I am told that a finish of beeswax and drying oil will prevent dye bleeding, but that's a question for another day).
4)  Embossed sheet metal facings, either over a leather facing or glued directly to the wood.
5)  Oil paint or encaustic (wax).  Unfortunately, I know of no evidence that these types were used in our period.

The first two leave us with the issue of the scabbard possibly splitting along the seams when the blade is pushed in or jostles inside the scabbard while the wearer is running.  Only the chape would provide reinforcement.  It's possible that small bands of organic material such as rawhide were tied or stitched around the scabbard farther up, leaving most of the wood bare for painting, but that's pure speculation.

Options 3 and 4 have good historical grounding, as the Issyk Golden Man's scabbard was covered largely with leather and small sheet gold adornments, while the Achaemenid scabbard from Egypt had a (now-vanished) gold facing pasted to the wood.  We're still left between a rock and a hard place:  Most reenactors won't have the skills, or money to pay someone who does have the skills, to make a full metal facing, since fitting one to the complex curves of a wooden core in the classic Achaemenid shape would require lots of careful hammering to expand the metal in certain areas.  On the other hand, leather alone is pretty dull-looking, and I've never seen evidence for Achaemenid scabbards with partial metal plates such as those at Issyk which would be easier to make.

I'll look into carrying out milk paint tests directly on wood.  In the meantime, Ivor at RAT brought to my attention the book Chariots in Ancient Egypt, which is said to have some evidence for painted leather.  Now I have a few more weeks of free time before the spring semester starts, I can finally start checking whether it contains anything potentially applicable to our situation.

Testing casein paint on leather, part II

Traditional gesso is sold as a powder, which is mixed with water in a 4:3 ratio, covered and allowed to soak for a day before warming it to melt the glue.  Warming produces a texture like melted ice cream.  The liquid is then painted on, allowed to dry, sanded, and painted on again.

After the first coat, I found that the remaining gesso cooled to a bubbly, rubbery texture that could be ground into the preceding layers, squishing it into a smooth paste in the process.  Both wet and dried gesso is a stark, eggshell white.

The next step was to mix and apply the powdered milk paint.  This is a sample of Real Milk Paint Co.'s "barn red," which has a slighly dull, ochre-like color (though if I use this type of paint on a large scale, I plan to use a colorless base and add storebought pigments so I know they're authentic).  I mixed it up in a common salad dressing-style cup with a lid and used another cup to measure out the water, as the recommended ratio of powder to water is simply 1:1.

Not long after the paint was applied, most of the gessoed swatches showed fine cracking.  Presumably the gesso absorbed water from the paint and swelled.  The swatches with glue but no gesso didn't seem to have this problem.

Interestingly, the swatches that were neither soaked in glue nor gessoed seemed to do very well, with no cracking even when I tried poking at them with my fingernail.  This may change over the following month if the curing paint becomes harder and more brittle.  The other factors are how well it handles water damage and whether the paint over chamois can be sufficiently sealed with wax if water becomes a problem:  a hard, non-sticky wax may also tend to flake off if the underlying leather is still puffy.

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!

Hide glue paint - the rain test!

Clockwise from bottom:  acrylic, hide glue with natural pigment, hide glue with natural pigment and food color, hide glue with pigment and alum, and hide glue with pigment, food color and alum.

 

After an hour's soak and a little rubbing.

 

Wow!  All the hide glue patterns washed out to the point of being barely noticeable, regardless of alum content.  Either I'm missing some information, or this is just to be expected - if historical paintwork was done in this way at all.

Hide glue paint tests - preliminary results

Achaemenid motifs on glove suede painted in acrylic (left), hide glue with added alum (top) and hide glue without added alum (bottom).

In the first batch (without alum), it took a water-glue-pigment ratio of about 5:3:2 to get the consistency I wanted.  I am just now realizing that I scaled the mixture down wrong and added twice as much alum to the second batch as I should have, which accounts for why it was somewhat thicker than the first batch (premature gelling - the alum content was probably around 3-4 percent instead of 1-2).

The thick consistency of the second batch - also probably caused by stirring up as much of the pigment as possible, as I noticed the first time that it tended to settle - meant that it didn't soak into the leather as readily, making for a blobby-looking design.  Since these test designs are very small, I think in more practical use it won't be much of a problem.  You can try different amounts of water and find what you prefer.

The acrylic paint is very slightly glossy, while the hide paint isn't, but I wouldn't say the acrylic looks too bad.  As for flexibility, the acrylic is far more flexible, but I think the hide paint has adhered well enough to the suede that it won't flake off.

Another issue is that the only pigment I had in the house was raw sienna, which when added to the glue, gave it a color almost identical to the leather itself - you can only see the faravahar on the top swatch because the painted areas reflect light differently.  This will make the last test hard to judge; I may try again with some food coloring added just to mark clearly where the paint is to begin with.  The last test is to soak the swatches and see whether the paints run; this will hopefully show us how our stuff will handle rain.  The acrylic paint manufacturer suggests "air-curing" it for one week before cleaning, so I'll be trying this out in a week and will probably report the results on Friday.

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?