Why do boards and paperback covers warp and curl? There is a simple answer, true in every case: moisture. Unfortunately that answer doesn’t really tell us anything: Too much moisture, too little moisture, when, where, and in what?
When board is manufactured certain levels of humidity prevail: Quite high at the start of the process and becoming progressively less as the board continues its travel along the machine. There are drying machines involved, and manufacturers seek to extract exactly the right amount of moisture. The factory may or may not be air-conditioned and humidity controlled, but the warehouse in which the finished product is stored is almost certain not to be. On humid days board will take up more moisture than it will on dry days. Even if the warehouse is humidity controlled, the board has to venture out on a truck for a journey to its final destination. Driving through the Arizona desert will give the board a different life experience than surviving a torrential downpour all the way to Florida. Of course the skids will have been shrink-wrapped in an attempt to minimize the fluctuation in humidity, but this isn’t totally airtight. One shipment of board will have had a different humidity experience than another. And of course this delivery of board will go into a bindery where things are dry, dry, dry, and that delivery will go into a sweltering steam box of a bindery. Books printed and bound in Asia merely have this problem in more extreme form as they go through many more changes in environment before sitting on the boss’s desk curling away.
One might hope that all the fluctuations would even out, and probably in most cases they do. But sometimes a batch will have been made humid, stored humid, shipped humid and cut up and bound humid, shipped to a humid warehouse, and bought from a humid bookstore by a sweaty reader. Or the opposite. Or a combination. I’m not aware of research which points to an optimum humidity profile, but I bet someone’s done it. Even if such a set of conditions were definable, it’s not practically achievable by anything other than random chance: climate variables are just too much for us.
Complaints about warping boards tend to hit New York publishing offices as spring becomes summer, and to be almost non-existent in the winter — though warping can be nicely induced by setting a book overnight on top of a hot radiator, something heedless editors will insist on doing. If it’s humid in the office and the board is relatively dry, as soon as it’s unpacked the book will start to take up moisture from the air. A casebound book with a laminated preprinted case has got only one thing to do when its board swells. The laminated cover prevents moisture penetrating from that side, while the endsheet doesn’t, so the board does what it must to grow. The side stuck to the laminated casecover will stay the same length while the other side will expand in length: result the cover bows. At the same time the spine edge of the board is held rigid while the fore edge can grow. The same thing happens with paperback covers: layflat lamination eases the problem, allowing moisture penetration from the printed side of the cover as well as from the inside, but it can’t get around the fact that the lamination will hold that side of the board tight while the other side can expand a bit. People have tried laminating the inside of the cover, which just increases the cost, makes the book hard to bind, and doesn’t remove the problem.
Another problem is that it’s almost always the case that warping boards are an issue with the office copies. You can’t really tell your boss that there’s no problem because all the rest of the books are packed tightly in cartons and are lying in a different humidity environment in the warehouse where they are going to be fine till they are shipped off to the completely different humidity environment that is the bookstore, or indeed the ultimate purchaser’s home. But that’s the truth, and carries with it the implication that we can’t actually control the situation when the customer takes the book home. Nevertheless we have to put the bindery though the exercise of explaining why the editor’s copy has warping boards, even if the answer ends up being the single word “moisture”.