The Abbey Newsletter

Volume 14, Number 6
Oct 1990


Letter

To the Editor:

Thank you for mentioning my talk regarding deterioration of glass plate negatives presented at the AIC, Meeting in Richmond. There are, however, two points that may require clarification.

The glass decomposition products found on deteriorated 19th-century glass plate negatives are not silanes, but silicates of various composition. At low relative humidity levels (below about 40%) these deposits are crystalline, and at higher RH levels the deposits dissolve. Organosilanes, on the other hand, are chemicals that have been investigated for the consolidation of flaking gelatin dry plates. For those readers interested in this topic, they may wish to refer to Sarah Wagner's excellent paper, "A Preliminary Study: Consolidation of Gelatin Glass Plate Negatives with Organosilanes," published in the AIC- Photographic Materials Group's publication Topics in Photographic Preservation, Volume III.

Regarding the storage of glass negatives, my research has indicated that negatives supported on chemically unstable glass, such as some collodion plates, should be stored at below 40% RH. At higher relative humidity levels the moist deposits result in silver image deterioration and flaking and softening of the binder and varnish layers. However, tests showed that the binder layers of some gelatin glass plate negatives may shear off the support when the RH drops below 30%. Therefore, the National Archives is now recommending 35% RH 2% as the set point for storage of mixed photograph collections that include gelatin glass plate and collodion negatives.

Constance McCabe
Senior Photograph Conservator
National Archives

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