Didnt the guys in finishing notice any heat and you would have thought that going through a couple of other processes someone would have said something.
If the paper is on the alkaline side say 8.00 ph or higher in combination with the acidic fountain solution and with the bio based inks that have a high concentration of mag and cobalt dryers and the sealing varnish you have the perfect storm for heat generation. When anything oxygenates it gives off heat.
In the closed container there is no were for the heat to dissipate and it just increases in temperature. It could quite possible get to the point of self combustion. If there were any caustics in the paper this could also aid in the temperature increase. Caustics and acids combined love to make heat.
This same problem can occur when wet or damp hay, straw or silage is placed in a mow or silo they can self combust.
I saw something similar once, the job was printed, AQ coated, folded and boxed, Upon delivery to the customer the boxes and contents were hot, and the entire job was now bricked (coating).
Samples were sent to some experts, it was discovered that an exothermic chemical reaction caused the problem.
It does contain mag and cobalt driers.
This one one of the three listed chemicals tert-Butylhydroquinone MSDS
MSDS - 112941
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