Sticking Together. Steven Abbott

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?!

      Figure 2.3 The carboxylic acid groups, CO₂⁻, on the casein interact with the calcium ions, Ca⁺⁺, from the lime to provide strength and water resistance.

      A different, and abundant source of protein suitable for glue came from the blood from slaughterhouses. Haskell, a Michigan businessman, had access to large volumes of blood from the Chicago stockyards and plenty of cheap timber to stick together with the “blood glue” to create Haskelite (what we would now call plywood) for vehicles, canoes and aircraft.

      There is one more protein adhesive with which I have had personal experience. Applying gold leaf is an ancient craft, and egg white albumin (a protein) is a key ingredient of the adhesive. I needed to decorate a harpsichord and was taught by an expert how to apply the leaf. I followed all the tricks that had been handed down over many generations; for example, huffing onto the albumin adhesive just before applying the leaf to make the adhesive slightly more tacky. I suspected, however, that many of these traditional steps must be unnecessary, and I worked out my much smarter way to do it. When I tried out this “smarter” way, the results were a disaster, so I swiftly reverted to the proven methodology.

      Adhesion is not just about sticking two or more surfaces together; it can also be about protecting a single, specific surface, in the form of a sealant or a varnish. An intelligent mind like Leonardo da Vinci's would not have been happy to see one of his paintings leave his studio without a hard, clear protective coating to help it survive life in a draughty palace or cathedral. His paints were oil paints, dispersed in solvents such as turpentine. He needed a tough, compatible polymer coating that could integrate itself into the paint as it solidified. There are such polymers available, in the form of amber, shellac, gum Benjamin and others, that are soluble in such solvents and thus provide the right compatibility. The problem (and it is the same today) is that any solvent good enough to unite the two might eat too much into the paint layer and destroy it. The artist had to find blends of solvents that had the right “bite” into the painting – not too much (destruction) or too little (the varnish falls off over time).

2.1 WHY BOTHER WITH ADHESIVES?

      Civilizations can get by without large-scale adhesive use. We know this because although the Egyptians, Romans, Greeks and Chinese had large-scale adhesive industries for, say, furniture making, medieval Europe coped OK for centuries without them, having lost many of the technologies and not having an urgent need to re-create them. You can make timber structures with holes and pegs, you can melt, hammer and weld metal components together, you can sew, lace, hook and tie clothing together. For decorations you can probably make some small amounts of sticky stuff that do the job, without the need for a significant industry.

      It wasn't until the 16th century that European princes wanted fancy cabinet making and laminated woodwork, and musicians wanted large, delicate instruments. The demand from the elite ensured that the art and science of adhesives was redeveloped. The first large glue factory (with horses as a key raw material) was founded in Holland in the late 17th century. In the 18th and 19th centuries patents for fish and casein glues were published.

      A kind researcher at the British library tracked down for me the earliest known British patent for a glue. The inventor, Peter Zomer, was from the Netherlands, and the patent is really about getting both the “train oil” (whale oil; the drops are seen as being like tears, which are traane in Dutch) and a fish glue from the Greenland whaling industry:

      British Patent #691. Whereas His most Sacred Majesty George the Second … bearing the date at Westminster, the Twenty-third of May [1754] I Peter Zomer by petition humbly represent to His Majesty that I had found out and invented “A Method of Extracting and Making from the Tails and Finns of Whales, and from such Sediment Trash and Undissolved Pieces of the Fish as were usually thrown away as useless and of little or no Value by the Makers of Train Oil, after the Boiling of the Blubber of such Fish, a Sort of Black Train Oil, and afterwards of Making from the Remains of such Tails, Finns, Sediments, & Undissolved Pieces a Kind of Glue called Fish Glue.”

      Not long after, the relatively sophisticated nature of the French adhesives industry was described by M. Duhamel du Monceau in The Art of Making Various Kinds of Glues, 1771. This fascinating book was translated into English in 1905 by the J. Paul Getty Museum and is easily found on the internet. Of special interest is that a particularly high-class fish glue (as opposed to the rather poor stuff made by Zomer) was available only from Russia and M. du Monceau took the trouble to find out what it was. In modern language, it was the swim bladders – rather, pure collagen – of beluga sturgeon.

      There is also an example of how a good source of glue (collagen) became a poor one through market forces. Those who fancy any form of “good old days” using “natural” adhesives produced by happy