Humidity and gum

Humidity has a huge effect on how gum behaves, in my experience. I'm convinced that a cool humid climate is perfect for gum, but that may be only because my own gum practice is calibrated for that climate. On the rare occasion when the wind shifts to the east and the humidity plummets, my usual gum practice suddenly doesn't work. The gum coating seizes up as soon as it's brushed on and refuses to smooth out, and the usual exposure times are suddenly much too short; even tripling or quadrupling the normal exposure times leaves the gum underexposed.

My experience is borne out by printing industry and technical literature investigating this variable. A graph of data prepared by the Lithographic Technical Foundation in 1956 showing the relationship between humidity and hardening in dichromated gum arabic shows a strong and linear (almost vertical) relationship between humidity and units of exposure required to reach a standard degree of hardening. It's no wonder that 3 or 4 times times the exposure isn't enough when the humidity plummets, as it did this week from 96% down to 17% and back up to 96% within the space of less than two days. According to the graph, it requires not 3 or 4 times, but 8 times the exposure to reach the same degree of hardening at 15% humidity as is required to achieve the same degree of hardening at 96% humidity.

Most people who live in drier climates than mine add some liquid to their coating mix to make it easier to coat.

I believe the high humidity here may be at least in part responsible for the fact that I can use the photoflood (EBV) bulb with very short exposure times, whereas others have reported much longer exposure times for these bulbs.

I've prepared a troubleshooting page about coating problems related to low ambient humidity.

Katharine Thayer