Window surround

[JanieB]

This is a question on behalf of some friends who are renovating a 17th c building. One of the original windows was removed in the past and replaced with a plate glass one. They are required to replace this with one like the others in the building. These have rather ornate stone/cement surrounds.

How would they make a mould of one of the existing surrounds? I thought hessian and plaster of paris, but someone may have a better idea.

They also have to replace all the uPVC windows with wooden sashes, much to their disgust but my delight.

 

[inwood]

A good stonemason should be able to replicate any existing stone on new replacements without the need for a mould.

 

[chuckey]

Please do not attempt to used plaster of paris and hesian. Proper moulds are covered with a mould release agent to stop the plaster sticking to the mould. Do a drawing, and for funny sections, get a piece of cling film, on the back of it slap on a decent thickness of car filler (such as "Plastic Padding") and puush it hard into the intricate part of the original, tape/prop up for an hour. Pull the whole lot of and do what you want to with it. Its a sample of a section at point "X"
I cast in situ a 12' long concrete lintel with a radiused chamfer in 4 places. The concrete was dyed to match the original Ham stone lintels, and it looks OK.
If you are proficient woodworker, build you own mould, get the internal surface as smooth as possible and wack on 3 coats of paint, then some thing like a wax polish (mould release). Mix up various samples of the concrete (say 1 litre of fine ballast + .25L of cement) with various amount of a suitable concrete dye - label them!. When the samples have gone off, make a judgement of what the best % dye mix is for your application. Mix more then enough for your actual job. Make sure that you vibrate the mix well to get rid of air bubbles adhering to the formwork. Be prepared to wreck the mould to get your lump out.
Frank

 

[JanieB]

Sorry Inwood, much as I agree with you, moneywise this is not an option.

Thanks Chuckey, I'll pass that on. The surrounds have been painted in the past and, rather than remove the paint, they will be repainted, so the colour of the cement/plaster is irrelevant.

 

[Gervase]

I'd echo what chuckey suggests. The vibration is vital to settle the concrete in the forms - you can hire a vibrating poker for a few quid from your local hire shop.

 

[BMcD]

Is it possible to have some pictures of what you are wanting to replicate, as there are various methods which can be applied depending on the profile of the moulding

 

[JanieB]

I'll try and take pics round the scaffolding. AND despite being a techy, I've never mastered posting pics. A new experience.

 

[NT]

Why would one need to use car body filler rather than plaster of paris & hessian? Either will do the same thing afaik, as I suppose would a much weaker cement mix. All would need a mould release agent of course.

Plastic film as a release agent leaves a fair bit of surface grooving, but these grooves can be filled with a finger and some more mix once set.

It sounds a bodge option, but if you dont have the money for stone then you dont.


NT

 

[Penners]

If you're going to make a mould of any parts of the original, don't forget that you can only do this successfully if the original has no undercut to it. If it has, then the mould will be damaged (or worse still the original could be) when you try to get it off.

This rough sketch will illustrate what I mean:

 

[NT]

Thats addressed by cutting the mould


NT

 

[Penners]

Quite right, but if you're not familiar with the problem, then forewarned is forearmed.