Adhesives, part 1.
This one’s not just for the bookbinders! I recommend starting with this bit on what makes something archival for a little background. When I began trying to lay out something simple about adhesives per a request we had from a printmaking class (ages ago, they’ve probably figured it all out by now), I realized just how much is involved. What follows is, I hope, a simplification of a huge amount of information, but something that might help you choose the right adhesive for your project.
A little vocabulary: traditional adhesives are either vegetable-based (pastes) or animal-based (glues). Synthetics eventually replaced the animal adhesives in many applications sometime in the 20th century, so those are called glues too. And bookbinders, you’ll want to pay close attention to whether an operation calls for paste or glue or a mixture thereof (and hope the author was paying attention to that too). “Paste out the board” means something very different than “glue out the board” when all parties agree to that distinction. The family tree above gives you a vague classification of the adhesives—methyl cellulose being the outlier as a vegetable-based adhesive but a relatively modern one compared to wheat and other pastes. More on that later.
In the conservation (and industry) literature, tape is often referred to as pressure-sensitive adhesive, which referrs to the idea of applying it by pressing it onto a surface rather than, as in the case of liquid glue, brushing. Tape consists of one of the above kinds of adhesive plus a carrier–-the plastic or paper part.
Down to business: animal glue. Here we’re talking about bone glue, hide glue, and gelatin, as well as some you might not have thought about: fish glues (isinglass), rabbit skin glue, egg white (albumin), milk (casein), etc. In your basic animal glue, the collagen (proteins) in the skins and bones of horses is extracted through a cooking and liming process, resulting in a transparent brown liquid when hot, and solid (pellets, usually) at room temperature. Gelatin is a more refined version of the same thing, closer to clear when liquid. You might have heard of egg whites, another protein, being used in tempera or other paints as the binder, or for gold tooling as a (weak) adhesive. Rabbit skin glue, processed in the same way as horse hide, has been a traditional adhesive for instrument making and sizing canvases. Isinglass is made by soaking the swim bladder of an appropriate fish (often sturgeon) in water, then heating it to release the proteins, and straining the solids away. Casein is one of the components of milk; to turn it into an adhesive the milk is curdled, and the curds treated with an acid to precipitate the casein, which is then purified and dried. It’s been used as a size in papermaking, as a stiffener of textiles, and as the binder in paints for a long time.
Protein is complex enough to keep the chemists occupied for a while, but for our purposes what’s interesting is that it can be super sticky. These glues make strong but brittle bonds: look at a falling-apart old case-bound book and you’re sure to see the yellow-brown animal glue on the spine cracking apart from itself and the paper. As conservators we like the stuff, compared to synthetic glues, when we need to remove it, because it swells (gets soft) easily in water. It is possible for some of these protein adhesives, particularly casein, to cross-link (form more and more chemical bonds to itself over time) and thus be less responsive to water or other attempts to soften and remove it. When would we use it? The more purified gelatin or isinglass would usually be used instead of generic animal glue in conservation, although bookbinders still slap bone and hide glue all over the place, and we’d choose to use it where we need a water-reversible adhesive (i.e. not PVA) that’s stronger than paste (also water-reversible). Examples might be where a particularly stiff spine is required, or in the repair of parchment, or, diluted a bit more, to re-adhere pigments flaking off an old manuscript.