G307 DCG Formula
(initial entry by John Pecora)
This is Jeff Blyth’s formula for plates that exhibit an increased sensitivity to the green wavelengths 514 and 532. I have been exposing between 5 to 20mJ/cm^2 with very bright results.
First it is necessary to coat plates with an emulsion of just gelatin and water using your preferred method (veil, bar, spin etc.) followed by curing of at least 12 hours. After curing the plates can be stored indefinitely, preferably in the refrigerator and just a few can be sensitized only when needed.
To sensitize the plates, make up a solution as follows;
Weigh out in order:-
- 0.5g glycerine
- 0.3g Aluminum Sulfate or Alum
- 100 ml distilled or de-ionized water
- dissolve everything up then add:
- 5g potassium dichromate.
Chill the solution to 5 degree C and then pour the solution into a container just larger then your glass plate.
At this point a red safelight needs to be used and the standard yellow bug light for DCG will not work.
Tilt the container to the side slightly, now take the glass and place it in the container, putting the edge in the deeper end of the liquid first and laying it down all in one motion while leveling the container. You will see the liquid slide across the top of the plate evenly.
Only leave your plate in the solution for about half a minute then shake the plate free of droplets and wipe over the glass back with a paper towel.
Stand the plates up against a wall at an angle and gently blow with a small desk fan or a hair dryer that has a cool setting until they are touch dry. At this point the alum in the formulation starts doing its job and is hardening up the coating a bit but it is only a gradual step and it is not instant.
Now your plates should be used as you normally use them, needing to be shot at once or refrigerated for later use.
AMENDMENT: Storage note about this solution If possible always try to use freshly made solution. However it can be stored in the dark in a fridge for a few days. The glycerine content will degrade rapidly if left in a bright light and slowly degrade in a dark reaction in the fridge. It may be possible to rejuvenate it with more glycerine but it is unlikely to be as good as the fresh stuff.
Post exposure treatment
You need to have your oven at 100C (212F) and have a flat clean metal plate in it so that the metal instantly and evenly heats up your dry exposed plate.
I have found that for a 4x5 plate 2 minutes is a good starting point. Depending on hardness of gelatin and exposure energy you may need to adjust this time. If the plates come out milky, increase the time. For the brightest hologram keep the baking time as low as possible without the plate coming out milky.
After baking at 100C take the plate out and put it immediately onto a cold metal surface to rapidly and evenly cool the plate.
Once the plate has cooled to room temperature, process with water and alcohol just as the standard DCG would be processed but use DI water in a container as the KDi will be lost into this water. When you are finished Sodium Metabisulfite can be added to change the CrVI to CrIII which is more environmentally friendly. You may need to leave the plate in the water rinse a bit longer to get all of the potassium dichromate out depending on the thickness of your emulsion. I use two baths, one for the majority of soaking to get most of the KDi out and the second as a quick final rinse. Once you have CrIII it will then be able to be precipitated out in a saturated solution of your sodium carbonate.
An alternate drying method would be to just hit the plate with the hair dryer or other blower for just a few seconds to get the majority of alcohol off but then put the plate back into the oven at 100C until completely dry.
Questions and Answers
Q. What qualifies as "curing" for newly coated plates? 12 hours at any particular temp or humidity ?
Well, the key is to have the gelatin cured enough to remain on the plate and not dissolve in the sensitizing bath. I would coat and leave in positive flow bench for 4 - 8 hours. At this point I would put the plain gelatin the fridge for later sensitizing and use. 12 hours is the minimum and as it is plain gelatin, storing for days or weeks is no problem. I believe 12 hours would be adequate for most environmental conditions.
Q. At what point does the sensitizer become light sensitive? Should the mixing be carried out under a safelight?
Yes. Anytime you add KDi or AmDi to glycerine or gelatin you should use a safelight.
Q. What happens if one leaves the plate in solution more than 30 seconds?
The only negative I can see is the gelatin will start to dissolve.
Q. Any chemists out there have any suggestions for a "preservative" to limit the glycerin dark reaction?
Why do we want to limit it? It is the dark reaction additive that gives the G307 its increased sensitivity.
Q. Why is a cold plate used after post-exposure baking?
This is used for larger (4x5" and up) plates so that the emulsion cools uniformly. Without this you may find that the brightness varies from the center to the outer edge of the plate due to differential cooling.
Also, to cool the plate immediately such that the baking time can be more precisely controlled. The post baking is the additional hardening. If the plate was allowed to cool to room temp without a cold plate, obviously the hardening from the baking would be extended. Vary the room temperature and the control time is lost.
Q. What does this mean? "Once the plate has cooled to room temperature, process with water and alcohol just as the standard DCG would be processed but use DI water in a container as the KDi will be lost into this water." Soak the plate first to dissolve out the KDi?
Yes, after the baking hardening there is still some unused KDi that will dissolve out. It is best to dissolve this out in a container such that it can be neutralized before disposing.
Q. "An alternate drying method. . ." Alternate to what?
Alternate to the standard DCG method of drying which is with forced hot air after the last alcohol bath.
Q. What processing regime have people had good success with? Combinations of alcohol %, fixer, etc.
DCG processing can be on A Beginners Approach to DCG, MBDCG and The Mechanics of Gelatin in the Dichromated Holography Process. It's unclear if or how the process might be changed for for G307.
There is no fixer used in the G307. The G307 is different only up to the water rinse bath just prior to the alcohol dryings. All papers and techniques to change bandwidth or color from the water bath on, for standard processing, should apply.
Q. When using Sodium Metabisulfite to make the CrVI more environmentally friendly, how much should be used? How can one tell when one has added enough to change the CrVI to CrIII?
CrVI is orangish in color. CRIII is bluish. When the solution has turned blue, that should be enough. A little more is better then not enough. I always add till blue and then some extra. Sodium Metabisilite is inexpensive. Unless you know exactly how much KDi is in the solution, I am not sure an exact measurement of Sodium Metabisulfite can be found. I wonder if there is a test to check if there is any CrVI left in the solution...???