Investigation into the use and effects of a Q-switched Nd:YAG laser for the removal of ink from paper
A Q-switched Nd:YAG laser is used to remove ballpoint pen ink from 'Roma', a 100% cotton rag art paper. The laser was operated at 1064 run, pulse duration 87 ns, and using a nominal fluence of 41 ± 12 Jcm-2. By shortening the pulse length (63 and 26 ns), using a Glan-Thompson prism the ink removal threshold was reduced to 24 ± 7 and 20 ± 6 Jcm-2 respectively. The physical damage to the paper is observed under SEM and found to be less disruptive to the surface fibres than contemporary mechanical methods of ink removal; scalpel and eraser. The tensile strength of laser treated paper is measured and found to remain unaffected below ink removal threshold fluences, although there is an increased tendency to tear at laser treatment sites. Repeat testing with artificially aged laser treated samples shows a marked decrease in tensile strength in comparison with an aged reference. Laser treated paper (nominal fluence of 41 ± 12 Jcm2, pulse length 87 ns) is tested for evidence of thermal degradation, via oxidation and depolymerisation. The tests include the Russell effect test for the presence of peroxides (oxidation), the methylene blue test for the presence of carboxyl groups (oxidation), FTIR spectroscopy to determine the presence of carboxyl, carbonyl, and conjugated groups (oxidation), and GC/MS to test for the presence of sugars (depolymerisation). The tests suggest that there is thermal degradation predominantly via depolymerisation. This is supported by repetition of the tests on laser treated samples prepared in an argon atmosphere. The laser treated paper is subjected to accelerated ageing in a humid oven to simulate natural is considered to be a reaction between the sizing agent (gelatin) and sugars formed during depolymerisation The tests are repeated on 'Roma' paper laser treated with shorter laser pulses (nominal fluence 24 ± 7 and 20 ± 6 Jcm-2, pulse durations of 63 and 26 ns), and this is found to have significantly less physical and chemical effect on the paper. Accelerated ageing of short pulse laser treated samples did not result in colour changes with respect to reference samples, indicating a more limited chemical interaction with the paper.