I thought this sounded like an innovative approach to stone restoration.  Refers to some ornate limestone carving work on a 19th C building here in Australia.  Written by another stone consultant for whom I have much respect.

"... the problem may be that the gypsum plus 
old silicone crust will be too hard to get through and we might have to gommage it off but then the limestone carvings might fall to bits ..."

So, what is the answer?  Responses cheerfully awaited.

Cheers

david

PS  For those who need a definition, Facade Gommage is a company of French origin which developed a method of abrasive blast cleaning soft stones without creating too much surface damage.  They work in Paris, and also in New York I think, as well as various other locales.