MITRA Forum Question Details

EditDeleteIn the Nineteenth Century, artist press pastels against the glass in frames and some used loose cotton behind the pastel to maintain the pressure. The cotton was a great addition, since it meant that the pastel that transferred to the glass, did so in an even manner. This meant that when the pastel came out of the frame, its appearance was not really altered, since pastel has so much more pigment than is needed. The cotton may have also served as a climate buffer, moderating the RH behind the glass and lowering the chance that morning sun light could pass through cold glass, without warming it, and warm the moist paper behind the glass, driving enough water out to form condensate on the glass. That case, is the most extreme, but whenever the relative humidity is high enough, mold spores can begin to grow and feed on cellulose and the binder in the pastel, whether it is touching the glass, or not . Thus, contact with the glass may result in the art being exposed to higher RH, when the glass is cold and sun light goes through it, but it seems impossible to conclude that the contact, between pastel and glass, caused the mold to grow, rather one can say that it may have enhanced conditions for growth. What our ancestors got right, with their cotton pressed framing, was a condition in which the pastel (which is hopelessly loose) that came off, did so without pastel coming off the art and settling on another part of the art. That last case presents a real hazard and suggests that when pastel is framed (as it should be to keep dust off) it be done with a low or no static glazing material spaced no more than 1/4" in front of the design, to keep the kind of static air cushion we see in thermo pane windows.

EditDeleteThank you, you have raised an intriguing issue. I am not aware of this practice among artists - if anything pastels have traditionally not been framed in direct contact with glass due not only to the increased chance of mold growth, but also to the extreme friability of the pastel medium. I have seen many instances of mold growth in pastels, especially in the darker colors which probably generate more heat as they absorb light (radiant energy). It seems likely that high humidity levels are easily formed in micro-climates between the glass and the pastel layer. The binder in pastels has usually been thought to serve as a nutrient. Glass has usually been used asa glazing material in order to avoid static charge generated by wiping rigid acrylic sheet (Plexiglas). However, now there is a new acrylic glazing product, Tru Vue Optium, that is non-static. Nonetheless, a "breathing space" must always be provided when framing up any works of art on paper. Sanded papers will not contribute to the problem of mold growth in pastels - as long as a space is provided between the medium layer and the glass or non-static acrylic glazing that will serve to dissipate trapped humidity. Hope this help!

EditDeleteThe two moderators that have weighed in on this matter ARE well-respected conservators with extensive experience in the field with international reputations. You are welcome to your own opinion of course and free to frame your pastels in any manner you would like but others would be wise to take note of the advice given above.