Regarding high spots, I've seen Blue Amberols that had some shrinking and distortion of the outer surface, but without associated splitting or chunks of plaster missing. Perhaps one cause of the high spots could be slow shrinkage of the skin over the decades, with corresponding compression distortion of the plaster core, like the way mountains were formed when the earth's surface cooled (a crude analogy, but this is what comes to mind). Depending on how uniformly or unevenly the forces exerted on record occurred (heat, humidity, UV exposure, etc.), the internal distortion of the plaster could be equally uniform or selectively uneven. For example, fifty records in a box stored for years in an attic would have had the outer layer of records absorbing the brunt of the extremes, and the outermost surfaces of the outer layer of cylinders, at that.
All this is purely guesswork, but this is what makes sense to me. Andy Baron On Apr 26, 2006, at 8:45 AM, Ron L'Herault wrote: > This seems to make a certain sense but I don't understand how/why the > plaster should develop high spots. If the plaster is expanding from > absorbed water, I would think that the rate and amount of expansion > would be fairly equal over its entire inner surface. I.e., all the > plaster would be expanding, reducing the inner diameter equally. What > other factors am I not taking into consideration? Gray spots may just > be the result of the plaster contacting a less polished/more abrasive > area of the nickel. If plating came off from rubbing against the > plaster, the entire surface of the spirals should get gray where they > contact the surface of the mandrel, right? > > Ron L
