Plaster of Paris has Ammonium Carbonate in it I know. It will decompose in air to form ammonia and carbon dioxide. Older homes often used homemade concoctions of plaster and all different recipes were used just like old milk paint recipes. Animal glue was used for bonding too and that had derivitives of ammonia.

Does your home have plaster ornementation or plaster crown molding? If latex molds were used to form them, during the process they may have soaked up a lot of the ammonia that is in latex. Latex is compounded with ammonia and it is part of the curing proccess with Plaster of Paris used in latex molds.

In the old days they would add ammonia to plaster to delay the curing process or add alum to speed it up depending on what the weather was on that particular day. These guys who worked with plaster and put it down on the 'lath and plaster' walls were artisians and adapted the recipe to their needs as they mixed it on the job. You may have a house where the plaster was mixed with a lot of ammonia to delay the curing on a particular day. You did not notice the smell unil it became damp from the water damage.

Also if there was a leak present before you moved in the PO may have patched the damage with Plaster of Paris adhesive patch plaster, and the Ammonia is being released from it now.

Another thing to look into is if the home has milk paint on the walls. Also, though milk paint is always thought to have been used on rustic homes and furniture, caesin paints did become commercially available in the 1900's. Milk paint was often used to do the art designs that was so popular in Arts and Craft homes of the era. There may be artwork painted on the walls using milk paint that has since been painted over. Milk paint had casein as a binder, which is hydrolysed with alkalis such as ammonium carbonate.

I know that those older vintage manequins that were cast in plaster, usually smell like urine if they are stored in a damp area, the same with the old vintage plaster church icons used in side shrines.

If your roofing guy said older homes smell of ammonia when they get wet he knows what he is talking about from his experience dealing with them. I haven't had the problem you describe in my own home but I do own a vintage 3' plaster cast figurine and it stinks like urine. It seem to smell worse on warm days. I bought it at an estate sale and I realized it stunk during the drive home with it in my closed up hot car. It did not seem to stink when I saw it on display for sale. I finally stored it in the attic as the smell really bothered me a lot. If you Google 'art plaster' I think you may find some into that could relate to your home.

Hope this helps.

Hi,
My retired union plasterer buddy told me that "retarder" is made from sheep dung. Fresh/wet brown coat plaster (like structolite) always has an earthy "wet dog" smell IMO.
Since it's a chimney there's the possibility that there is bat guano or bird droppings in it that has gotten wet and now emits an ammonia smell.
Casey

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