Correcting Milky Holograms Milkiness is caused by the film being too soft. There are many ways of fixing this problem (making the emulsion harder) but increasing exposure is not the best way. It sounds as if the gelatin you are using is naturally soft. So let's look at some other ways to use your gelatin and get it harder.
You could try any of these or combination of these with a shorter exposure time.
1. First, shorten your exposure let's say by half (10min). Then after the exposure hold the hologram in your hand by the edges and with the laser shutter open, hold the hologram in the laser light and keep moving it side to side. The idea is to get additional crosslinking and thus hardening without actually creating any more fringes. Do this for the remainder of time of your original exposure so do this for the 10 minutes you took off the original time. If you cut your exposure to 5 minutes then do this for 15 minutes. After some testing you may be able to cut this additional lighting time down.
2. After a shorter exposure, 5 or 10 minutes, and before processing put the hologram in a lab oven (do not contaminate your cooking oven) and bake it at about 120F (49C) for 3 to 5 minutes. It's best to have a piece of flat steel in the oven to lay the hologram on, so when you put the hologram in the oven, it heats up all at once. Then when you take the hologram out of the oven, place it on anther piece of flat steel to cool it evenly.
3. Prior to exposure do the above baking technique. Then let the hologram cool and stablize to ambient room temperature and humidy before shooting.
4. Decrease the amount of water in your homemade "Fixer" (pyrosulphate/alum/water 20g/4g/400ml) and increase your fixing time. You'll have to test the increased fixing time as you never stated how long you fix. With higher concentrated formula, you may not have to increase fixing time at all but a little testing will tell. If you get too long, greater then 2-3 minutes, then decrease water. It's best to keep fixing to about 1 - 2 minutes. Try 20g/4g/300ml.
5. Put a couple of drops of Glycerol (glycerine) in your original emulsion formula. This will help speed up dark reaction in those 24 hours which will essentially cause the film to be a little harder.
6. Age your plate in a very dry area for 2 maybe 3 days instead of 1 day. You could make a few plates and shorter expose a plate each day and see what happens as the plate ages.