Monday, July 31, 2006

Polyfilla and Old Painters

If I had to name a single favourite construction material, it would probably be Polyfilla, a plaster for filling up holes and cracks in walls and ceilings. With regular gypsum powder it would be difficult to add exactly the amount of water which would make the paste stiff enough to stay in the crack, and after a few minutes all the paste you mixed would harden nearly instantaneously leaving you very little working time. Furthermore, gypsum shrinks when drying and thereby forms new smaller cracks which you will have to plaster up a second or third time. But then someone, apparently at Polycell UK, thought of adding cellulose to gypsum powder. Mixed with water the resulting paste is very pleasurable to handle, it hardens less dramatically and shrinks less. Where I have needed a little harder finish, for example in an outward corner, I've successfully added small amounts of cement.

Polyfilla is nowadays a worldwide brand, but recently I learned that old painters have used the basic idea for centuries: they cooked oat meal in water and mixed the resulting thin porridge instead of clear water to the gypsum powder the next day. Instead of oat they might also use other sources of polymeric carbohydrates, such as mashed potatoes, mouldy bread, old newspapers, etc. Supposedly a pair of old underwear has been spread in the walls of an architecturally renowned house in central Helsinki.

If slightly cheaper and tougher finish was required, the gypsum powder would be mixed with coke slag. This material, colloquially called "kananpaska" (chicken shit) or "karhunpaska" (bear shit) was the predominant material for building light walls in Helsinki: a mold would be erected on one side of the future wall, the wall would be constructed layer-by-layer by throwing the gypsum-ash paste on the mold or previous layer, the mold would be removed, and a finishing ashless layer would be plastered on both sides of the wall. The demeaning name probably follows from the messiness of the substance when demolishing the wall: the coke slag produces extreme amounts of slowly setting dust. When local coal heaters was replaced by district heating during the fifties and sixties this construction material has fortunately lost popularity.


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