Mold coating is the process where you use spacers between two pieces of glass, one with a release agent, in order to get an even coating thickness. Depending on the exact chemistry desired a coating thickness is set by the spacers. A place to start is 7um. Swollen unhardened gelatin can contract on hardening by a factor of 8, but partially hardened stuff can be less than half this.
- Take two identical pieces of glass. One with tape on two opposing edges. Different tape thicknesses will yield different final emulsion thicknesses.
- Treat the piece of glass without the tape with Rain-X to prevent sticking.
- Place a small puddle of emulsion in the center of the plate with tape on it. Make sure the emulsion does not have any air bubbles in it. With practice a known about of emulsion can be used that covers the plate but with minimal excess and overflow. #Then gently lay the piece of glass without tape and with the Rain-X (Rain-X side facing the emulsion) on top of the puddled glass as flatly as possbile. You will see the puddle of emulsion expand though th entire plate.
- Place 4 black paper clamps, two on each side right over the tape.
- After the gelatin has dried, minimally 3 or 4 hours, gently pull them apart. A very flat surface is left behind.
- I would add that air bubbles in the gelatin can be removed using the edge of piece of adsorbing paper. I found bubble formation is a quite common problem when making plates. - Cristiano
Tips from Dave Battin
The glass I use for my mold is 1/4" mirror (back silver), while I won't call this glass optically flat, it is of optical quality.
The Rain-x is the key, I put a couple drops on the mold, then buff it in, heat the mold and film plate, and meter out the emulsion. To see the emulsion do a little dance on the rain-x is really amazing!
I have a "two suction cup" handle that I use to hold my plate and slowly drop/ hinge the plate along the taped edge of the mold. I then place six larger paper clamps around the perimeter
My plate size is 5"x 15" and i can get 3 nice 4x5s per mold , If metered carefully (absolutely leveled mold) ,only a few drips will come off the mold ends ………….
As soon as the gel sets (a few mins) I move the still slightly warm plate and mold, on to a frozen piece of granite, where it rapidly cools down. Buy the way the granite can work both ways, for both cooling, and heating! I purchased these as granite tiles a the local Depot (12"x12"), and cut them up ........
The tape I used is plain Scotch Tape and is .0025” or @ 63 microns thick.
And to my surprise the film thickness came in @ .0004” or @ 10 Microns.
The gelatin was Knoxx brand at a 15 grams /110 ml DI water, poured @ 70+degrees C.