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Testing of historical varnishes and lacquers

Testing of historical varnishes and lacquers

Testing of historical varnishes and lacquers

Dear forum members,

I am currently working on a 4-year PhD-project on the characterisation of historical varnishes and lacquers of the West (Northern Europe from the 17th century until the 19th century).
These coatings consisted mostly of 1 or more natural ingredients such as resins, gums and (essential) oils.

The aim is to reproduce a selection of varnish recipes and characterise changes of the mock-up samples before, during and after artificial ageing. We expect the layers to lose elasticity throughout the ageing process (due to increased polymerisation but also due to the loss of volatile plasticisers). Furthermore we expect that the layers become more and more hydrophillic.

Of course we will not test all physical and mechanical properties, since we do not have enough time to do so. Instead, we are mainly focused on the change of previous mentioned properties because we assume they have a direct impact on the conservation of these varnish/lacquer layers on furniture, etc.

On this matter, I have a few questions concerning some of our measurement techniques:

  • For the measurement of elasticity we use the TQC Pendulum Damping test (König, Persoz).
  • For measureing changes on a hydrophilic level we wil use the Theta Lite Optical Tensiometer.
  • Changes in gloss will be measured by the Elcometer 408 (Gloss- and DOI-meter)
  • Changes in scratch resistance will be measured by the Pencil Hardness test.
My questions are:
How does this test cope with aged surfaces that are having surface defects such as cracking (craquelé)? If this has an influence, how to cope with this so that these measurements are comparable with the measurements before ageing?
  1. What is the influence of brush-marks on the test results?
  2. What is the maximum surface irregularity in order to assure the measurement is valid? (I will produce mock-ups on a plywood substrate. Although it has not the tendency to warp it still might be the case that this will happen after the artificial ageing).
A rather similar question: since UV-radiation often leads towards a change of surface gloss (surface topography changes due to embrittle, sometimes comparable to blooming effect), how can the contact angle measurement in such case be comparable to the measurement before aging (when the coating has a very high gloss). I experienced that a powdery surface of course will influence the contact angle.

And a last question: after the sessile drop for the contact angle measurement is placed on the surface, how long do you wait before commencing your measurement (I was said that a delay of 15 seconds is often used, to avoid the measurement of irrelevant data).

Thank you in advance,

Vincent Cattersel
University of Antwerp
Conservation Sciences

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