Rubber positive for grotesque 137

The rubber positive of this keystone is cured and out of the mold now, I had JUST enough rubber to make it as the 2nd photo shows how thin the corner is, but with the plaster backing I will be adding it will be good.
The original this was molded from I removed from a building around 1977 that was at 1 West 111th St in NYC on the corner of 5th Ave, the buildings that were there have been replaced by this new one.

Now I need to set up to cast the 4″ deep plaster backing for this.

I like this pourable rubber, it’s also a lot less expensive @ $105 for 2 gallons than what I use for brushing-on molds, and for these use once items it doesn’t need to be premium rubber.
The big issue is being pourable it’s really only suitable for relatively small or thin plaques because it would take a lot to fill larger or deeper pieces.

Now I have the backing form made for this, I’ll have work to do to it yet.
Making the wood form to pour the plaster in was a bit more involved than other pieces as the bottom of the keystone is arched upwards as this was originally over the top of an arched window. I had to shape a piece of scrap wood to fit the curvature.

If I use some clay on the sides to get a texture, smooth and fill gaps etc the plaster form needs to be damp and making the plaster molds’ 4 side pieces has to be done while the clay is still kept moist so the clay doesn’t shrink.

Once the 4 sides are made it can sit until I get more plaster to finish it- sometime the end of June since I only have 50# of plaster left right now and it’s pretty sure it won’t be enough to also pour the largest section over the face.