Gesso

[rebeccac]

Anyone made up any gesso plasters?

We buy gesso in Italy and after leaving an opened bag in a damp house for 3 months I tried mixing it with water like I do with lime to make putty.I found that after 3 mixes it stopped taking up the water and became very much like a lime putty.I painted a thin layer over a wall that had been half lime plastered but top half original (gesso?) plaster.I'm going to finish with eith limewash or clay paint.

Just wondered if I was missing anything?

www.mybruzzohome.blogspot.com

 

[MdB]

I'd never heard of Gesso plaster so looked it up.

First link suggested it was calcinated gypsum, others that it was plaster of Paris. Looking up plaster of Paris told me that the two were one and the same thing which surprised me. I was under the impression that PofP was a good thing to use in renovation - now I find out that it is actually gypsum, I'm not so sure. Can the experts enlighten?

 

[plasticpigeon]

Yes I always thought plaster of Paris was gypsum. I don't know why modern stuff is pink. Maybe the aggregate or additives, it seems harder than plaster of Paris. I'm not an expert though! One interesting thing is that plaster of paris expands slightly as it dries while lime shrinks. This makes p of P good for filling up gaps/cracks, or for finishing off round fireplaces as when it dries there will be no shrinkage around the fireplace or whatever. Also in my house I can see that the fire surround was stuck on the wall by inserting metal loops in plaster of paris both in the fireplace and the wall. Unfortunately the fireplace was long gone, but the metal loop was still in the wall when I took the new plaster off. It's also used for running cornices. I spoke to a guy at SPAB a while back and he said lime was great for "breathability" as it shrinks as it dries leaving a very small void round each piece of aggregate. Therefore the structure is like honeycomb so any absorbed water can evaporate quickly. Gypsum expands as it dries so doesn't have the same physical structure. It is still very good at absorbing water though maybe not so good at letting it go!. Maybe some other people can shed more light on the situation or tell me if this is baloney..

 

[stuart45]

Air can pass through lime mortar taking the moisture with it. Plaster does not allow this so much, but soaks up moisture like a sponge.
Gypsum is calcium sulphate with 20% water, and when heated to drive off most of the water it becomes PofP, or hemi-hydrated gypsum.
As it sets too quick retarders are added to make it more usable for plastering. I used some PofP for a small repair last week and it went off as soon as I got it on the wall.

 

[middi]

Gesso = chaulk ( in English)

 

[rebeccac]

I have picked up lumps of rock in Italy which locals call gesso and I understood these rocks were burnt to produce gesso particularly in the Abruzzo region although I think artists use the term to describe a mix they prime canvas or wood panels with.

Gesso/gypsum doesn't set so hard if you slake it as described in my first post.I can keep using it for several hours at least.Also it can be slaked,dried in blocks then reconstituted to use as a primer without it setting in mins.

www.myabruzzohome.blogspot.com