DEAR TIM: In the log books of our historic light station, the lighthouse keepers mention whitewashing, painting, and something called calciuming. I have tried in vain to discover what that might mean. All I know for sure is that it's something they did to the interiors of their dwellings. The keepers' houses are brick and the interiors have horsehair plaster directly on the brick walls, and plaster on the framed interior walls. Can you tell me anything about calciuming? What was it? Why do it? How do you do it? Should I still be doing it? Ellen Henry, MFA, Curator of Collections and Education, Ponce de Leon Inlet Light Station, Ponce Inlet, FL
DEAR ELLEN: As much as I hate to say it, I believe the term calciuming might be peculiar to those particular lighthouse keepers. It probably is a slang term for a particular type of whitewash or plaster treatment applied to the inside of the walls of their dwellings and possibly the actual lighthouse.
Because of the extremely harsh marine environment, they needed a wall treatment that would not disintegrate. The constant humidity and heat in your location would quickly cause regular finish materials to fail in short order.
The biggest clue is the word calcium that happens to be the first part of calciuming. Calcium is the principal element in lime. Lime is, of course, the principal ingredient in whitewash, older mortars and plaster. Lime is created by heating very pure ground-up limestone. The chemical formula of limestone is CaCO3. The heating process disturbs the chemical makeup of the limestone and drives off carbon dioxide. The resulting powder is CaO or quicklime and is chemically unstable.
The lighthouse keepers could line the inside of the dwellings with limestone rock, but something tells me they wanted a smoother finish. What's more, installing solid rock walls is costly and labor intensive. But, if you systematically apply layer after layer of thin liquid limestone that hardens on interior walls, you get a smooth surface, it is hard as rock and it requires minimal skill and labor.
Limestone is a very hard and durable rock. It can withstand tremendous punishment from Mother Nature. By reversing the chemical reaction once you have quicklime, you can make man-made limestone. Quite possibly this is what the lighthouse keepers did.
The quicklime wants to react with other chemicals so that it can once again be stable. The humidity in water vapor, regular liquid water and carbon dioxide in the air around us readily satisfy the quicklime's desire for the lost oxygen and carbon that left the limestone when it was heated.
So if the lighthouse keepers regularly mixed lime with water and possibly some very fine sand, they would be periodically adding layer after layer of very thin durable limestone to the inside of the lighthouses. I think this is what was happening and when you stop and think about it, it makes perfect sense.