Patching Up Victorian Plaster

[Regent Dave]

I have a late Victorian house (circa 1900).

I need to patch up an area of plaster approx 6"x10".
I've taken away all the very dry / crumbled plaster & the edges of the area are reasonably solid.

Having read some of the existing threads, I understand why I shouldn't use a modern gypsum plaster for patching up lime plaster.

Do I assume that it is lime plaster on a circa 1900 house?
How can I verify this / What should I look for?

Is there a "How to guide" on the site somewhere that will help me identify the correct product to use for repairs?

 

[Ian Bond]

Lime plaster contains coarse sand, with a range of grain sizes, and perhaps animal hair, in addition to the lime. It can often be removed easily from the wall, so great care is needed.

Gypsum plaster is usually dense, with a uniform consistency, unlike the varying sand grains and tiny pebbles etc in lime plaster. It can be very difficult to remove from a wall, unless it gets wet and falls off / turns to mush. Lime plaster's far more resilient to being wet.

Both types of plaster usually have a fine finishing coat or skim. The gypsum skim is brittle and cracks very easily. If you skim the walls in old house with gypsum, small movements in the structure can cause large cracks in the finish. Lime plasters are more tolerant of small amounts of movement.

Here's a repair guide for lime plaster:
http://www.mikewye.co.uk/mikeplastering.htm

There are a number of other lime suppliers, and you may be lucky and find one nearby. They should be able to offer advice too.