This may be of help or not....In the book " The Darkroom Cookbook" by Stephen G. Anchell on the chapter on Stop Baths and Fixers he say's"...when sodium carbonate, one of the most commonly used alkali in developers, comes into contact with acid, carbon dioxide gas is released, which can cause blistering in the emulsion of both film and paper. The problem is more critical in film, where it is exhibited as a PINHOLE, usually in dense areas of silver deposits, such as sky...
Hope this helps Cheers Tim Mackrell
