A Beginner's Approach to DCG Holography
By: John Pecora (Note: as this is an original article please do not edit it unless you are John. Please use the discussion page to comment on this work.)
As the availability of green lasers becomes cheaper and more widespread, so does the potential for one to make their own dichromated gelatin (DCG) film and holograms. The purpose of this paper is to provide a basic step-by-step set of procedures such that the beginner may have success in producing their own DCG film and making simple DCG Holograms. There are many variables in the fabrication and processing which alter the aesthetics of a DCG hologram, most of which will be beyond the scope of this paper. And the basics here will not guarantee a professional quality hologram but will lead the reader down one correct path to successfully make DCG film and DCG holograms. It will be up the reader to take the next steps to perfect the quality and repeatability of the DCG hologram process.
DCG emulsion is made from a simple solution of ammonium (or potassium) dichromate, raw gelatin and water. Exposure to the DCG emulsion is done in the Green or Blue with higher sensitivity to the shorter wavelengths. Processing is simply a soak in Standard Photographic Fixer followed by a soak in water followed by a dehydrating process in one or more alcohol concentration baths. As a DCG hologram is susceptible to image loss when exposed to moisture, the DCG hologram will be sealed. There is a multitude of ways to perform each of these functions and the ones presented here have been tried and prove to work but may not be the best or suit a particular application for the DCG hologram.
Also, it is very important to insure safety at each step of of the process using good chemical safety practices, knowledge of the chemicals and equipment being used, proper disposal of chemicals, and common sense. It is not within the scope of this paper to point out safety hazards and it is the responsibility of the reader to research each and every potential safety hazard. I also suggest reading the entire paper first to familiarize your self with the procedures and note any materials and supplies you may need.
Cleaning the glass properly is important. If the glass is not cleaned properly the emulsion can lift off the glass in spots or completely during processing. Also, if there are any dust particles, the emulsion tends to have different properties at that area and a circular ring of deformation of the hologram will be seen around that area. Soak the glass in a 3% concentration of hydrochloric acid overnight. This can be bought as muriatic acid from most home improvement centers. You could also use a 25% concentration of household bleach. This procedure also works for recycling glass from previously coated plates, but I found the bleach takes longer. After soaking, using rubber gloves, scrub the plates with a plastic wool scrubby used for cleaning Teflon pans. Steel wool may scratch the glass. After scrubbing rinse the glass thoroughly under running water and place in a tray of running water. Then repeat the rinse process while rubbing again with the plastic wool. After the final rinse, lean the plates against the wall on a paper towel. Before the plates dry completely use a paper towel to dry off one plate at a time and continue to turn the paper towel until the plate is dry. You will hear and feel the difference between a damp plate and a dry one. Repeat for the other side of glass. Do not touch the plate with your skin or oils will be left behind which can also cause the emulsion not to stick to the glass.
DCG Emulsion Fabrication
DCG emulsion is comprised of an amount of distilled water, dry gelatin and ammonium (or potassium) dichromate. A good starting formula is 100:12:3 for the procedures described here. Take the water and place it in a heat resistant glass or plastic container. Place this on a magnetic heater/stirrer or in a double boiler. Add the gelatin to the water while it is cool and allow it to mix for a couple of minutes. If you are not using a heater/stirrer the stirring should be done by hand. Bring the temperature up slowly to a maximum of 120°F and a minimum of 110°F. Once the solution reaches the 110°F temperature, continue mixing until the gelatin mixture is completely dissolved. With the heater/stirrer allow the solution to be well mixed the entire time but not so fast as to cause excessive bubbles or foam. By hand, mix well for one minute every 5 minutes (this get laborious by hand). Mixing too long is better then under mixing, and I suggest 45 minutes after the minimum temperature is reached for a more aesthetic hologram. But again shorter times may be used as long as the gelatin is dissolved. It will look very clear and not cloudy when dissolved with no suspended particles. From this point on, a safelight must be used until the after the water bath in processing. A good safelight to use is a standard yellow incandescent bug light. Now add the dichromate. Allow this to mix until it is all dissolved (about 15 minutes) within the same temperature range. When this is completed, filter the mixture through a paper coffee filter into a clean container. A funnel or similar can be used to hold the coffee filter paper. It is best to allow the narrow end of the funnel to touch or be very close to the bottom of the final pouring container such that dripping from the funnel end does not produce bubbles. The container can be a beaker or other similar container that can easily be poured from but at the same time can be put back on the heater/stirrer or back in the double boiler to maintain the previous temperature range. If the emulsion is cooler during coating the final emulsion thickness will be thicker. Take a Q-tip and pop or remove any small bubbles that may be on the emulsion. The emulsion is now ready to coat. The emulsion can be stored at this time in a refrigerator, but should be sealed, labeled, and not allowed to be exposed to light.
The glass can be coated at room temperature or warmed up to the temperature of the emulsion (110 - 120°F.). The warmer the glass is, the thinner the final emulsion thickness will be. A great starting point is room temperature glass (70°F). Take a cleaned glass plate and blow it off with a shot of canned air to remove any dust particles. Hold the glass with one hand at a 45 degree angle over a heated excess tray. Use gloves. You can use a therapeutic heating pad to keep your tray warm. This keeps the emulsion from hardening in the excess tray. Slowly pour the emulsion up the left side of the plate about ¼ inch from the edge, then across the top and down the right hand side. If this is done correctly, you will notice a nice even flow across the plate when you are pouring across the top. Then take a ¼ sheet of paper towel and ball it up and wipe about a ¼ inch of emulsion from the bottom edge of the plate in one continuous motion. Take the plate and immediately place it on a table and spin it as 78 RPM. An old phonograph works great and has been used for years by many holographers. Let this spin for 10 minutes. A helpful suggestion is to make a rack that can hold plates horizontally on the spinner but leave a nice air space between the plates so they can dry evenly. Then one can coat and spin many plates at a time only stopping the spinner momentarily to quickly load the next freshly coated plate. I have found that spinning longer is better and I allow my plates to spin for at least 10 minutes after the last plate has been loaded into the spinner. If a spinner is not available then lean the plate against a wall in a tray that can be use to catch the excess for thinner emulsions or lay the plate flat and horizontally for a thicker coating. I prefer the spin method. If you run out of emulsion in the pouring container, just pour the warmed excess back into the pouring container keeping the pouring container at an angle so as not to cause any bubbles. After the plates are coated the excess emulsion can be poured back into the container and stored in the refrigerator for a later coating session. I have found the emulsion will keep for weeks this way.
Exposing the Plate
The plate is best after 4 hours old and can be up to a week(s) old. I have found the brightest holograms are between the 4 and 12 hour age. It seems when plates are older they are harder to get broadband replay and/or replay into shorter wavelengths and lose some sensitivity. The simplest recording geometry is a Single Beam Reflection in which the object is lying down on its back and the plate is laid right on top of the object. Make sure the object and plate do not wobble. Place the emulsion facing the object. As DCG is quite relaxing in the energy requirements I suggest doing test exposures with the times being doubled thus covering the largest range of times in the least amount or test exposures. Once the time range is found with your laser it will be easy to reproduce. An example is 10 seconds, 20 seconds, 40 seconds, 80 seconds, and 160 seconds. After exposure allow the plate to set in complete darkness for 2 to 5 minutes before processing.
Processing the Plate
All temperatures can be at room temperature (70°F). Take the plate and put it in Kodak Rapid Fixer with hardener. The Fixer should be mixed as per the instructions for the most dilute mixture (paper 1:7). Gently rock the tray until all yellow is gone then an additional 15 seconds. This should take anywhere from ½ minute to 2 minutes. I use a white tray to observe the yellow more easily. Once this is completed, place the plate in running water for 5 minutes (a tray of water can be used if running water is not available). I now turn on a quartz halogen light that shines on the spot where I will lean the hologram to blow it dry. Then take the plate and place it in 35% alcohol for 15 seconds. Then 70% alcohol for 15 seconds, then 91% alcohol for 15 seconds then finally 100% alcohol until diffraction is visible (anywhere from 15 seconds to two minutes or longer). As soon as diffraction (colors) is seen, allow another 15 seconds in that bath. Then take the plate out and lean it against the wall in the overhead light. With practice you will find which angle the diffraction is seen in the light and which way that relates to the visibility of the hologram when blow drying it. As soon as you lean the plate against the wall begin blow drying it with a hair dryer set on its hottest and strongest settings. Blow dry very close to the plate. Start at the center and in a circular motion move to the outside of the plate and repeat often. If the plate is leaning the right way the diffraction and image should start to get really bright. Continue drying until hologram in completely dry. You cannot over dry but you can under dry. This usually takes me 5 minutes minimum.
Sealing the Hologram
If the hologram is acceptable in quality and brightness to your liking, it must be sealed against moisture. After it is completely dry, use a razor to scrap off ¼" of emulsion from around all four edges. Three edges will be easy if you maintained ¼" when pouring the coating. The bottom wiped edge from coating will probably need the most attention. Now have another cleaned piece of glass ready the same size as the hologram. Mix up some 5 minute 2-part epoxy. I use a Q-Tip with the swab cut off. Now take the Q-Tip and use it to lay down a bead of epoxy around the entire cleaned edge on the emulsion side of the hologram. Take the clear cleaned piece of glass and place it over the hologram. You should see the epoxy sandwiched between the glass plates at the edge where the emulsion was scraped. Look closely and make sure there are not voids where the epoxy did not get sandwiched. Let the plates dry horizontally and check often to make sure the top plate does not slide and move into a different location. After about 15 minutes the hologram can be displayed as liked.