Sticking Together. Steven Abbott

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      Figure 2.2 The tough, insoluble collagen from bones, skin etc., (left) needs to be boiled sufficiently to break it down into soluble glue molecules (middle) without being broken further into smaller lumps that constitute gelatine (right).

      It is a genuinely difficult challenge to get these processes to work at all – imagine having to provide the quality control to ensure that they work day after day!

      One useful additive to cartilage-based glues sounds vaguely amusing to us now: urine. It is at first surprising how often urine appears in ancient recipes and processes. We now know that the main chemical in urine, urea, is one of the few molecules that interacts strongly with proteins such as collagen to make them more soluble. Camel urine was for centuries a hair-care essential because it allowed the keratin protein in hair to be made flexible before being shaped into whatever was the current fashion. Urea is frequently used in skin cream formulations and is an essential part of “natural moisturizing factor” created by our skins. As with all complex products, adding too little or too much urea would cause problems; it would be interesting to know what quality control procedures were used to ensure consistent urea additions from such a variable raw material.

      How do we know what sorts of glues were being used a long time ago? Archaeology shows that locals used the resources to hand, and some of them are easy to identify even after millennia: Aztecs used rubber; Mesopotamians in 4000 BC used bitumen to attach ivory eyeballs to statues; and from the Neanderthals onwards, the plentiful supply of birch bark in Northern forests drove the use of pitch. In South Africa, Yellowwood pitch did the same job 60 000 years ago. Collagen has a distinctive protein “signature” – its mix of peptides is very different from the majority of proteins. If you come across some decorated Chinese burial staffs from 3500 years ago and have access to a modern proteomics lab it is not too hard to find some bits of hardened adhesive which, despite some degradation over time, show the distinctive collagen signature. Carvings from ∼1450 BC in Thebes in Egypt show glue pots being used for laminating veneers; analysis of Egyptian pot fragments suggests, again, collagen glues.

      Those artisans who made furniture fit for Egyptian pharaohs needed adhesives with the right consistency to keep the chairs and tables from falling apart during the hot, dry periods when the wood and glue dried out, and during the hot, humid periods when the wood was swollen and mould and bacteria would feast on the peptides and proteins in the glue.

      Similarly, for the Greek and Roman nobility, an absolute “must have” was furniture that combined ornate wooden surfaces with at least some degree of practicality. With the right collagen glues, it became possible to apply thin sheets of intricately patterned veneer to an otherwise dull table. To be even fancier, different coloured woods could be stuck together in patterns, the art of marquetry.

      The joints and veneers those Egyptian, Greek or Roman artisans worked on allowed them to easily spot any problems caused by the weather and, with their collagen-based adhesive systems, to make timely interventions. This is because their glues had a key advantage missing from our current high-tech versions: with modest amounts of heat and water or steam the artisans could get a joint to fail in a controlled manner so they could replace or repair it, returning the furniture to its former glory. To this day, violin makers use animal glues so that it is easy to take the instrument apart and fix any problems. Those famous Stradivarius violins will have been taken apart many times over the centuries while still retaining their glorious sounds. If someone used a modern adhesive, taking the violin apart to repair it would probably destroy it.

      In societies where milk was common, it was rather easy to separate out a protein, casein (the word is related to “cheese”), by allowing the skimmed milk (all the cream separated) to go sour or by deliberately adding an acid. Casein, like collagen, is a protein and on its own forms a solid film with some adhesion, but that's not how you make a good glue. In fact, the process is quite tricky. The casein has to be washed, dried and ground into a powder. Then it has to be stirred up – and here's the thing – if it is stirred up with lime it makes a great, water-resistant adhesive, as long as you can act fast enough before it sets solid; if it is stirred up with sodium hydroxide it has a long pot life and gives a strong bond but not a water-resistant one. The right mix of lime and sodium hydroxide gives you a good working life and adequate water resistance. That doesn't sound so hard, and with internet access to recipes and easy purchase of pure lime and pure sodium hydroxide it really is not so hard. Now transport yourself back in time where it wasn't even clear what sodium hydroxide actually was. And no one knew that lime contained calcium ions that love to bind strongly to carboxyl groups on the protein (Figure 2.3). These simple, natural glues are neither simple nor natural. Who first came up with the idea of mixing casein with lime and sodium hydroxide – and was able to successfully reproduce its excellent performance?!