DCG holograms are very sensitive to re-adsorbing moisture. If a hologram disappears after bieng in humidity you can usually get the image back by reprossing the hologram in alcohol drying baths. If you want to make sure the hologram is permanent you will need to seal the back side.
The two most common methods are to seal the back with a glass plate or to coat the back with a cyanoacrilate adhesive. Most art holographers use the glass plate method.
== Glass Plate Method 1 - Full Coverage (UV Epoxy) ==
When sealing a hologram with the glass plate method it is important to scrape at least 3mm of gelatine off the edge all the way around the hologram. This insures that the edge of the gelatin is sealed. Once this is done the epoxy is spread evenly over the entire emulsion and a glass plate is place over the sealant. The entire sandwich is place over or under a UV light (black light) to cure. This sealant is bought to have the same index of refraction of glass and should dry clear. This sealant can be expensive. If you wish to make your own UV sealant see Jeff Blyths "Do It Yourself" UV sealant below. == Glass Plate Method 2 - O-Ring (Standard Epoxy) == An ecconomical approach that works very well is to use 5 minute two part epoxy (at any hardware store). Scrape 3mm of gelatin off the edge all the way around as indicated above as best as you can. Clean the glass cover plate. Mix the two parts of the two part epoxy as directed on the epoxy label. I use a q-tip cut in half and a piece of scrap glass. Once the epoxy is mixed use a tool, like the q-tip rod to evenly spread a bead of epoxy around the scraped 3mm area on the hologram. Place the cover plate on and insure there are no place missing any epoxy by visually inspecting it. Place on level surface and let dry. '''Here is a post from Jeff Blythe on making UV cure epoxy at home:'''
===A DIY UV sealant===
In keeping with the grand DIY philosophy of the Forum I thought I would put down some basic ingredients for making your own out of materials which are fundamentally cheap because of their big industrial use. However before that a hypothesis that fits observations I have made. I believe that the reason DCG has been so notoriously difficult to seal up and prevent moisture getting in is not necessarily due to any fault of hydrophobic glues being somehow rather more moisture pervious than expected. I believe the real trouble has been that sandwiched between 2 glass sheets the gelatin layer contracts with age and builds up a significant vacuum. This results eventually in outside air getting through microcracks inspite of diligently thick glue having been applied around the edges of the sandwich.. This contraction effect might be just to do with the basic properties of the gelatin under prolonged lighting but it could well be more to do with the final stubborn traces of water /alcohol still hanging about and alcohol vapour can very gradually (we can be talking “years” here) make its way through the edge sealant increasing the vacuum effect. Anyway whatever the cause an obvious way to minimise it is to put the newly processed DCG in a really dry warmer I am not sure what temperature is best but 60-70C for as long 24 hours seems to work or alternatively I have left them in a really effective desiccator for a week. Then without giving the ultra dry DCG a chance to re-absorb ambient humidity a dry glass cover plate with dry sealant can be put on. This topic has been discussed on the forum before and some of you guys have had vastly more experience than me at sealing. To make a UV curable sealant,you need a monomer, crosslinker, and free radical generator for UV. Monomer : Methyl methacrylate or better (more hydrophobic) is butyl methacrylate (NB. Not tert-butyl methacrylate) Crosslinker: Ethylene dimethacrylate (alternative silly name by Sigma Aldrich is ethylene glycol dimethacrylate). UV sensitizer (free radical generator in UV.): DMPA or dimethoxyphenyl acetophenone. One can use about 1 part DMPA to 100 parts monomer to 5-10 parts crosslinker. It gets harder the more crosslinker you add of course. (~80-100% crosslinker just cracks up). But the mix has initially a rather low viscosity , lower than the commercial stuff. It is a good idea to store mixture over silica gel in a fridge in the dark. Jeff