Shellac in plaster mouldings to make them bendy?

[Craig]

Hi - an experienced engineer just mentioned something interesting to my wife and I, and I wondered if anybody had experience of this?

Our house has relatively elaborate cornice mouldings at the ceiling level, which consist of projecting acanthus leaves, often several inches wide, mounted on a bead.

Where we have had to replace individual leaves, we have taken a cast in plaster of Paris and then glued them individually in place with slip.

This chap said that originally, the mix included shellac to make the mix flexible, and a run of mouldings could be pressed around corners/curves.

Our decorator would love to try this, as we have a whole room where the mouldings are gone - but we don’t really have enough info to start. Anybody know anything more?

Many thanks
Craig

 

[BMcD]

I use shellac for sealing reverse moulds on a bench so as fibrous casts can be made several times over

To make a mould more pliable use a richer mix of lime putty to casting plaster

 

[chuckey]

My take on this is that you have to incorporate into your new cornice some plaster board joining glass fibre tape or such like. This way you can you can "bend" your sections right up to its breaking point and the tape will hold all the bits, so a rub down and a light application of slip will rescue a potential disaster. Making the sections thin would make them more flexible. I have seen TV progs about this sort of thing, one with Fred Dibnath springs to mind, he was visiting a plaster restoration work shop. More research required?
Frank

 

[Craig]

Thanks Frank though I can't see that being the option my decorator would like (hes a very particular man). These are leaves projecting out of a bead, so no where for the tape to go, rather than a "run" of cornice.

 

[NT]

I've never heard of or tried this method, but I cant imagine shellac adding any flexibility to anything, its quite the opposite of flexible.

What would be flexible enough to wrap around curved corners?


NT

 

[Moo]

If, as you say, the room in question has lost all its mouldings, giving you a blank slate, why not make a mould from one of the corners in the other room?

 

[hoppy]

Shellac wont make your mouldings pliable....we only use shellac as a release agent for plaster moulds.
You can either "Model" a mitre leaf in clay or take a "squeeze" of youe original using rubber and thixo.
You would be very fortunate to find any fibrous plasterer's who carry your design in stock, as these would have been moulding for your property specifically at the time.

Hope this helps
PDS..(See our listing)

 

[Nigel Watts]

I have never heard this and I dont believe it would work.

The way many elaborate cornice mouldings were made in the late 18th - 19thC was in two stages. First the straight mouldings would have been run in situ, and then the precast ornaments, made in quite short lengths, would have been stuck in place.

Today, of course, complex mouldings are much more likely to be made complete, off site, in fibrous plaster then fixed in place. The advantage of this method is that it is much cheaper than the old method, but it also has disadvantages.

The first problem is that in the second method the cross section of the finished cornice cannot be too thin. This means that many original designs which were run in situ cannot be accurately reproduced in fibrous plaster made offsite. Fibrous plaster made offsite is also very straight and regular. Old buildings usually are not. The in situ method by contrast allows the cornice to be built up to follow the line of the ceiling, to go around bays etc.

 

[MdB]

Note: this is a 2007 thread.