On Fri, 3 Aug 2018, Amanda Roberts writes: I've got some acid-free tissue that's been giving me some odd, fluctuating pH readings, and a paper conservator suggested I reach out to you. I'm working on a hat and shoe rehousing project and ordered a roll of unbuffered, acid-free tissue.
The readings from the pH pen are probably correct. The key here is that it is unbuffered tissue that is being tested. Unbuffered paper, that is, without any calcium carbonate buffering, naturally tends to measure around pH 5 to 6, even without the use of acidic sizes or other additives. Paper fiber is naturally slightly acidic. Why was the reading fluctuating? My guess is that this has to do with the way most printing and writing papers are manufactured today--heavily loaded with calcium carbonate. Calcium carbonate is cheaper than paper fiber and very opaque, so manufacturers today tend to load the paper stock with as much calcium carbonate as it will take--I've seen numbers as high as 16%. This is why the regular printing paper tested bright purple. When such paper is recycled to make paper for boxes and packing materials, the calcium carbonate stays in the mix, so even brown boxboard might test purple with a pH pen. If the unbuffered tissue spends time resting and rubbing against the calcium-buffered boxboard, some of the calcium might rub off on to the surface--and so you get a purple color from the pH pen. Paper from the interior of the stack will still test yellow. Not all boxboard is made from recycled calcium-loaded paper. Some boxboards will test purple and some yellow. If unbuffered paper is what is needed for the specific project--for example, cyanotypes, some architectural drawing types, silk, and some other fabrics--then a good quality unbuffered paper is the correct thing to use, even if it tests on the yellow side with a pH pen. "Acid-free" in this case doesn't mean alkaline, but just free of added acidic substances. * * * * * * * * * * * Shannon Zachary Head, Dept. of Preservation & Conservation, University of Michigan Library ****** Unsubscribe by sending a message to [email protected] Searchable archives: http://cool.conservation-us.org/byform/mailing-lists/cdl/
