EXPERIMENTS WITH HISTORICAL GLUE AND PAINT

Peter Beatson - NVG Miklagard



Introduction

Glues based on casein (the protein of milk and cheese) are attested in classical and medieval texts [1]. Exceptionally strong, permanent [2] and waterproof, they were used in carpentry up until the invention of synthetic adhesives in the early twentieth century. Casein can also be used as a binder for pigments, to make paints.


[1] C. M. Helm-Clark (2007). Medieval Glues Up to 1600 CE.
[2] Being based on proteins it is, however, vulnerable to biological attack unlike modern synthetic glues.



Preparing curds

Ingredients: Skim milk [3], white vinegar [4].

1. Gently warm 500ml skim milk to 35-40°C [5] in a glass or ceramic jug or bowl (not metal). Don't overheat.

2. Use a wooden spoon. While stirring, quickly add 100ml white vinegar. Lumpy curds will swiftly form (add a little more vinegar if they don't). Stir occasionally for a minute or so, until curd separation seems complete.

3. Allow curds to settle. Using a sieve, strain off and collect curds, discard liquid (clear/yellowish whey).

4. Return curds to jug or bowl. Rinse by adding about a cupful of diluted vinegar (one part vinegar to five parts water) and swirling briefly.

5. Strain off curds again and discard liquid. Once curds stop dripping, shake the mass out on to a scrap of coarse linen or cotton. Gather and gently squeeze out as much liquid as possible.

Yield: a rubbery squeezed curd 'cake' about the size of a squash ball (~50g), which can be kept in the refrigerator until needed.


[3] Powdered skim milk is more economical (make up as directed) but liquid skim milk can also be used for convenience.
[4] Cheapo “no name” vinegar is fine - approx. 5% acetic acid is all that is required.
[5] That is, lukewarm, about body temperature. Curds that form in cold milk are way too small to strain out easily, but take care - overheating will damage the proteins.



Preparing glue

WARNING - Lime causes severe eye damage, use gloves and safety glasses.

Prepare glue immediately before use. For big jobs, prepare and combine several small batches to make one big batch.

Ingredients: Milk curd (see above), hydrated lime (calcium hydroxide) [6].

1. In a ceramic bowl, crumble a small amount of curd - two tablespoons should make enough glue to bond about a square foot, depending on how thick or thin you make it.

2. Divide the curd as finely as possible using the edge of a spoon, or if it is dry enough it can be ground in a mortar and pestle.

3. Add a small amount of lime (1 tsp. of paste or ~1/2 tsp. of powder) to the curds. Work the mixture with a pestle or the back of a spoon. The curds will become sticky - keep working, adding small amounts of extra lime [7] and dribbles of water until curds have combined and dissolved into a smooth, white, gummy mass. Take your time and be thorough, or your glue will contain lumps of undissolved casein.

4. Add water a few drops at a time, rubbing it in gently (don't mix too vigorously, or it will get foamy), until the desired thickness is achieved - something similar to PVA woodworker's glue is okay [8].

5. Decant batches into your glue pot - cup or other container. It can be pressed through a strainer to remove any remaining lumps of casein.

To use - Casein glue is wet bond, it works best on porous surfaces (wood, leather) and for close joins only. Use freshly prepared glue, it can't be kept [9]. Coat both surfaces liberally, and press together while wet. Use clamps or weights to press the joint together while curing (12 - 24 hours). Clean up with water.


[6] Known as slaked lime or hydrated lime, as it is produced by burning crushed limestone to create calcium oxide = 'quicklime', then adding water. Get the purest and finest-ground lime you can, food grade (pickling lime) is the best - it can be bought at Indian food shops as a pure white powder or paste called chuna or chunam (it is normally used in paan, a southern Asian chewing drug). Builder's lime, lime plaster powder, and garden lime are mixtures with limestone and/or contain impurities and grit, I found them unsatisfactory. Quicklime itself should also work and was historically used, but is a much more reactive and hazardous chemical - I haven't tried it and wouldn't recommend it - if you do, wear protective gear and add slowly while mixing well to prevent explosive heating.
[7] The idea is to add the minimum amount of lime needed to convert the curd proteins back into their soluble form. Excess calcium makes the dried glue brittle and gives paints a milky bloom. Note - teaspoon (tsp.) = 5 ml; tablespoon (tbsp.) = 15 ml.
[8] The thinner it gets, the weaker the glue, but if it is too thick it can't penetrate into the surfaces to create a bond.
[9] Maybe half an hour - once the glue skins over or goes runny, its no good.



Preparing paint

Casein paint is brittle and thus not suitable for flexible surfaces. Paint can be applied directly to smooth absorbent surfaces such as wood or leather. Fabric could be pretreated by sealing with coat of thinned glue, or could be coated with gesso [10] if a smooth ground is required.

Ingredients: Strained casein glue (see above), powdered natural pigments.

1. Add a little water to the powdered pigment to create a thick paste [11].

2. Add casein glue to the pigment paste while mixing. Use at least an equal amount of glue and pigment paste. Two parts glue to one of pigment will make a glossier, more waterproof finish.

3. Use fresh. Apply thin layers of paint using short overlapping strokes, and allow to touch dry before recoating. Thick coats will shrink and crack.

Colour should not rub off once dry, but may become tacky and smear if wetted. For an additional waterproofing seal, rub a 2:1 mix of beeswax and raw linseed oil (or olive oil) over the thoroughly dried paint with a soft cloth, then rub off excess.


[10] A medium used to prepare artist's canvas, such as equal parts chalk and powdered hide glue.
[11] The archaeological record indicates that oyster or mussel shells were used for paint pots from prehistoric to medieval times.




LIST OF NATURAL PIGMENTS TESTED


Colour

Natural Pigment

Product name ('Brand') [12]

Coverage [13]


Red [14]

Red ochre (haematite, iron oxide)

Jeweller's rouge powder

Excellent


Yellow

Yellow ochre (limonite, iron oxide)

Italian yellow earth ('Rublev')

Good


Brown

Umber (iron & manganese oxides)

Cyprus burnt umber ('Rublev')

Moderate


White

Pipeclay (kaolin)

Kaolin ('Art Spectrum')

Good


Black

Carbon (bone char) [15]

Bone black ('Rublev')

Moderate


[12] 'Rublev Colours Dry Powder pigments' ($10 for 100g) and kaolin both available from Kadmium Art + Design Supplies (80b Bay Street, Broadway, NSW 2007). 'Rublev' brand by Natural Pigments L.L.C. (USA).
[13] Good to excellent - satisfactory results with a single coat. Moderate - may require additional coats to obtain even and intense colour. Results can vary with quality of ingredients, particularly the pigment's particle size will affect its hue, even mixing, coverage & penetration into the ground.
[14] Dark red-brown like dried blood, resembling “Falu red”, the natural iron oxide paint traditionally used on farm buildings in Sweden.
[15] Black from plant char was also tested ('Rublev' German vine black, aqueous dispersion), but it coagulated upon mixing with the casein.



Kite shield with leather facing, using historical glue and paints
Curved ply blank supplied by MFC.
‘Experiments with Historical Glue and Paint’,
written and webbed by Peter Beatson.

(c) Birka Traders 2014. Not to be copied without permission.

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