Stonework Moulding

[Peaky 556]

Hello all, I'm looking for any experience of making a mould for casting concrete into, to simulate a coursed stone wall face.  What I am trying to do is make the sides of a cutting look a little more interesting than just plain concrete!

I have a plan for the master plug, and am wondering if a silicon rubber moulding compound will be best.
The loose plan is to make a wooden box with sides tapering to suit the angle of my cutting sides, then affix the mould sheets to these sides and pour in the concrete into the outside gaps to the earth.  The sides will be quite steep, so it will be important not to have the mould too stiff or it will not disengage.

I've looked at general RTV moulding compounds on eBay, and several seem suitable for cement, but what 'softness' to choose?  There are Shore Grades A22 to A28 offered.  I 'suspect' that the lower figure will be the softer.
If the above sounds workable, I will also need a way of fastening the mould sheets to the wooden frame.
Thanks, Tim

[John Candy]

Tim,

If the wall is to be purely "decorative", I would recommend resin in preference to cement.
Not only will it be quicker and easier to cast but will not suffer from damp/frost damage and cement castings can be difficult to remove from moulds without fracturing.

I use only A25 silicon for moulds for cold casting which is pliable for easy release of castings but also has adequate rigidity for flat castings as well as moderately-sized 3-D castings.

The harder RTV I use is resistant to high temperatures for casting whitemetal and is not very pliable and prone to splitting if over-stressed.

I believe Roger McL ("Bolingbroke") has tried cement casting with not very satisfactory results, so suggest you seek his opinion.

For architectural/statue casting, there is a material "ciment fondu" of which I have a bag but have not tried. I also have a booklet written by an expert in its use.

Regards,
John.

[John Candy]

Here is an example of a wall I cast last year from resin (it has been outside all winter).

[Peaky 556]

John,
The cutting walls are structural in that they hold the earth back and allow footfall close to the edge.  There is also a large volume of concrete needed so I doubt resin will be economically viable.  It is not a matter of removing a fragile concrete piece from the mould, rather the opposite of removing the mould from concrete that has become part of terra-firma.
I am leaning towards a soft compound, as the mould sheets will be fully supported by the wooden framework. 
How soft is your A25?
Cheers, Tim

[John Candy]

Tim,

It is slightly less soft than the "Jigstones" moulds which Roger lent me but there is not a lot of difference.
The Jigstones (which are designed for cement casting) could be A20 but cannot be certain since I have never used it.

You could always cast the structural retaining wall in plain concrete and then attach a resin overlay (using "Gripfill" as the adhesive).

Regards,
John.

[Peaky 556]

 Thanks John.  The resin facing sounds a bit more work to me, so I might stick my neck out and buy some A15 grade silicon rubber for use with concrete directly. The best deal I have seen (and I need a good deal for experimenting) gives a choice of either A15 or A28.
Your wall does look very nice though, so how did you get the mottled ironstone finish?
Regards, Tim

[John Candy]

Tim,

I don't know the price of the materials you have found but I currently purchase mine from www.tomps.com at what I think is a very good price for the quality of product.
I bought for many years from www.tiranti.co.uk (and their products are excellent) but switched to Tomps about a year ago (Tomps are about 30% cheaper).

If you buy low grade silicone it will tear/deteriorate/chemically decompose very quickly and you will be repeatedly making new moulds.
A good quality mould will produce upwards of 50 castings : My "stone" moulds have now produced 100+ and are still going strong.

As an example,  2.1kg of Tomps "Value Range RTV" available in A20 / A25 / A30 is currently on offer at 22GBP + VAT (plus carriage).

Regarding my stone walls, I first paint with Humbrol No.67 dark grey matt enamel (or Revell equivalent) and then (when it has dried), wipe over with Humbrol No.26 light brown acrylic using a stiffer brush.  If the brown looks to have gone on a bit on the heavy side, you simply brush over (while wet) with a brush very lightly loaded with acrylic thinners (I use Tamiya X-20A thinner) and it tones down nicely.
It is very easy (and quick) to achieve the results in my photo.

Regards,
John.

[Peaky 556]

John
The supplier of RTV I have in mind is DWR Plastics, selling 2.08 kg including activator and postage for GBP30.40 (on ebay).  This comes in either A15 or A28 Shore hardness grades.  I am making up the master wall at the moment and can post pics when ready and the RTV arrives.  I have done nothing like this before, so there is almost certainly going to be a long delay before I get things just right!

When it comes to tunnel portals, I have had good advice they should be made as removable items, so I guess that the resin technique you describe will be most suitable.  Any chance of some advice on what sort of "resin' we are talking about, and where to buy it?

Regards, Tim

[John Candy]

Tim,

I use the same polyurethane resin for my walling as I use for the wagons.

My choice is Biresin G26 (from Tiranti) or (cheaper but just as good) Axson F19 (from TOMPS).
I need to make four single-track tunnel portals before long and F19 is the material I shall use.

John