The effectiveness of the pens may be affected by external methods. Simply having a banknote pass through laundry, depending on the soaps and bleaches used, can cause a bill to fail the test when it is otherwise accepted. Additionally, it was discovered almost immediately that changingtreating thea acidity of acounterfeit note,orwith mosta anydilute other piecesolution of paper,[[vitamin a napkin, typical typing paper, cardboard, et cetera, with lemon juice,C]] will cause a false positive,negative: that is, a place mat in a restaurant with dried lemon juice on it will respond to the iodine-based ink as though it were made of the same paper as a valid banknote.<ref>{{cite web|title=How do Counterfeit Pen Tests Work?|date=20 July 2017|work=Banknote World Resource Hub|author=Abdullah Beydoun |url=https://www.banknoteworld.com/resource-hub/portfolio/how-do-counterfeit-pen-tests-work/}}</ref>
