Thank you guys for your interest & questions. I will put another head on and do my upmost to answer them.
I must admit Paul you have raised a good point to concider,and it is one I pondered on too. That's why I added a little something extra to the Plaster of Paris Mix. A Secret Passed on by Word of Mouth in the Beano Family! The Moulds were left to dry out for many days until Rock Hard. The longer one leaves this stuff the Harder it will get,almost like Ancient Concrete,but not heavy,and the Moulds and what they produce are Waterproof too. When cured the casting,or infact a better description The Pressing once lifted slightly with a craft knife just peels off and out nicely without any tug or pull to the actual mould.
You guys are right,It is also possible to take an impression of the Mould completely in the same mix Silicone Rubber,and cast in plaster which on tiny casting is OK,but would break if dropped. Dalibor the main reason why the Silicone Rubber Mix , is once the flat inner stone work has cured it will bend into shape when the Arches are created. Whereas a plaster casting would not. The same applies with the roadway over the bridge,because I can have complete flat castings made that will bend into shape over the Bridge.
Below. Proof of the Bending
Inspite of all the bending it flips back and stays in shape .
I also intend to make Railway Tunnel Entrances in much the same way,and stone work and rock faces that will bend into the shape that I wish,and which will be stuck in place using Silicone HA6 Adhesive,much the same way as this project will be. Well I know it will stick,it`s made of the stuff. Simple wood supports inside each end ,and in the middle of the Bridge will make it easy to construct. I Hope!
I have many expensive moulding materials that I could use, but I want this to be the cheapest easiest way of making moulds that would inturn produce fine castings that are well fit for my purpose. If Hat Industries can produce such stuff I thought that I would take a leaf out of their book and do the same.
This Two Part Mould on the right hand side of the picture,was made with the same Silicone HA6 Mix already described,and it being much thicker is still flexible so the casting will just pop out with ease,it is also very tuff. This will be cast in Plaster of Paris and Dental Porcelain Powder Mix.
The stuff they use to make Teeth!
Using a different head I hope I have made complete sense. BB