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 artisans may not enjoy this little snippet from the book:
“The feet of oxen, formerly esteemed, are now looked on as one of the bad materials that can be employed, & especially since the Butchers have begun to carefully remove a tendonous part of them, called the small nerve, or the shin nerve, that they sell by weight, & rather dearly for the production of a kind of oakum which is useful for caulking the panels of carriages, or to make suspension straps for carriages. When the feet are thus stripped of this tendonous part, they produce only a mucilaginous substance which is not suitable for making good glue; & if anyone makes use of them, it is because of their low price.”
As M. du Monceau's book indicates, by the early 19th century, we enter the world of industrial adhesives that take up the rest of the book.
2.2 TECHNICAL BASICS
Before we start, we need to make sure we have the right basic ideas and language to understand what makes a good, or poor, adhesive system. Be assured that what follows is rather gentle, yet at the end you will be able to understand what's really going on when we stick things together. We take it step by step, with none of the steps being especially difficult.
The trickiest, final, section covers the science of the polymers used as adhesives. The emphasis is on the few important principles (e.g. what polymerization is, what a crosslink is) that anyone can grasp. Readers who dread chemistry need not worry.
2.3 UNITS
We have to agree on a few measurement units and technical terms. Most readers will be familiar with them as they aren't too exotic.
For length we will use metres, m, millimetres, mm, micrometres, µm and nanometres, nm, each 1000× smaller than the previous. The unit of centimetres, cm, doesn't fit into that nice scheme but is so common that it has to be included.
For time we will go down to µs, ns and ps, micro, nano and picoseconds, one millionth, billionth and thousand billionth of a second.
For weights and loads we will use kilograms, kg, grams, g and Newtons, N, which, if a weight is involved is just weight times gravity. For our purposes, gravity is 10 m s−1 s−1 so 1 kg is 10 N.
Force per unit area has the units of N m−² which is also expressed as Pascals, Pa (Figure 2.4). One Pa is rather small, so we often have kPa, MPa and GPa for kilo, mega and giga, 1000, 1 million, 1 billion Pascals.
Figure 2.4 Force, in N, is applied over an area in m². 1 N m−² is called a Pascal, Pa.
If you pull, say, a piece of plastic which has a cross-sectional area of A with a force F, then you will get a fractional increase of length (the change in length divided by the original length), ε (Figure 2.5). The force per unit area is the stress in Pa. The fractional increase in length, strain, has no units. If you divide stress by strain you get modulus which gives an idea of the strength of the material. Modulus is also measured in Pa, as strain is dimensionless. Dividing by a small number gives a larger number, so the smaller ε for a given stress, the larger the modulus, which makes sense because a stronger material will stretch less. The modulus of typical polymers lies in the 1–4 GPa range, steel is 200 GPa. We will find later in the book
