Dating Portraits - Card Thickness
The best way to measure card thickness is with calipers, but obviously not everyone has such a specialized tool handy. Most people do, however, have access to regular 20 pound bond paper, the kind of paper most typically used in copy machines, laser printers, and other computer printers (at least here the US). One sheet of 20 pound bond is about .004 inch, or .1 mm thick. Card mounts varied from about .01 to .05 inches thick, so we can work out a rough equivalancy:
To measure a card, place it on a flat surface, abutting a stack of three sheets of paper. Add paper a sheet at a time, and run a finger across the surface where the two come together, first one direction then the other. You can feel a distinct ridge on the higher side at first, continue until they feel equal.
Darrah suggests that cards roughly .010 to .020 inches (.5mm or less/ 5 or fewer sheets) date from 1858 to 1869.
Cards .020 to .030 inches (.5mm - .75mm/ 5 to 7 1/2 sheets) date from 1869 to 1887.
Cards .030 to .040 inches (.75mm - 1mm/ 7 1/2 to 10 sheets) date from 1880-1900.
Cards greater than .040 inches (1mm/ more than 10 sheets) thick date from 1890 to 1910.
I haven't worked out the figures yet, but my initial feeling is while cards do tend towards increasing thickness over time, the actual thicknesses may be a bit higher than the figures Darrah suggests, at least for the thinner cards. I find that with calipers it is possible to press them tightly, and get a thinner reading than is really accurate, because the pressure tends to compress the fibers of the cardstock.
© 2005 by Andrew J. Morris -- www.ajmorris.com -- All Rights Reserved