White fog on DCG - Dinesh February 3 2004 at 9:19 AM
(no login)
When I use the Great Lakes gelatin instead of the off-the-shelf, store bought Knox gelating I get a white fog throughout the plate. I thought I had some documentation on it but cannot find it.
Does anyone know what properties about the gelatin (bloom) and/or the hardener in the fixer or combination thereof that might be causing this. Those are the two variables I believe caused it. Is it the bloom is high thus hard and no hardener should be used in the fixer?
You don't say if this white fog comes on unexposed or exposed and processes plates.
You should read (or reread) paper on MBDCG + TMG.
Jeff Blythe explains that gelatin has 'yield point' that you must care to not exceed it.
Extract of : http://cabd0.tripod.com/holograms/id4.html
<...
Processing. It is very helpful to keep the following mechanical model in mind. If one hangs weights on to the end of a steel extension spring, then removing those weights will result in the spring recovering its original un-extended length unless the weight exceeds a critical value. When a weight is of that critical value, the material of the spring has reached just beyond what is known as the "yield point". When that weight is removed the spring would normally never completely recover its original un-extended length but will remain permanently distorted. Had the material of the spring been a little stiffer, then that particular weight would not have stretched the spring beyond its yield point. If the temperature of the stiffer spring were to be raised however, then that weight may again take the spring beyond its yieldpoint.
DCG in water behaves similarly to the stretched spring. At a certain temperature, the stretching forces due to swelling in water may take the gelatin beyond its yield point. Had the water temperature been lower, then the gelatin would not have reached its yield point.
The art of making the most noise-free and yet bright DCG holograms, is to take the gelatin as close as possible to its yield point without exceeding it. Stretching gelatin beyond its yield point creates noise in the finished hologram. Using the onion layer analogy of Curran and Shankoff, the holographic fringe layers might resemble dried out onion with crinkled-up layers if the yield point is exceeded. A sign that you have got the swelling just right is no milkiness in the exposed area but milkiness in the unexposed area.
...>
Sorry, the plate is nice and clear with a little yellow tint before exposing and after exposing. It is only after processing in the alcohol that it gets this overall haze throughout.
The temperature of my fixer, water rinse, 10%, 55% and 85% baths are 70 degrees F. My 100% bath is 95 degrees F.
I have not changed my processing technique or temperature but notice this white fog/haze when I use the Great Lake gelatin not when I use Knox. So it seems the springiness in the great lakes has less tensile strength, thus a lower bloom? Does that make sense? It is supposed to have the same bloom of the Knox which is about 225 - 250. There must be another property of gelatin that I am not aware of. I guess I could reprocess in a different way, like adding more hardener but I am used to using the Knox gelatin and since it is so cheap I will purchase some more
John,
Try keeping your 100% bath temperature at the same temp as the rest of the baths (i.e. 70F). When gel containing some water is put into a 100% bath, the water is heated up, and this extra heat seems enough in your case to take the gel molecules beyond the yield point. This suggests to me that your new gel has better uniformity than the Knox gel: the higher temp bath is taking most of the molecules beyond the yield point for the new gel, whereas it may only have been taking some of them beyond it for the Knox gel.
"Anonymous" makes a good point about the yield point of gel.
However, what is not usually clearly explained is that the level of base hardening of the gel ( ie the concentration and length of the hardening bath) affects the yield point of the gel. The more the amount of hardening, the higher the temperature at which gel molecules 'yield'. The level of base hardening of the gel is dependent on a large number of variables ( coating temperature, relative humidity of the storage environment, the age of the coatings- to name but a few). All this makes finding the yield point of gel like shooting at a moving target. This is why it is essential to try to change only one variable at a time when experimenting with DCG.
"Anonymous" was me
I never use an hardening bath with MBDCG but I remember that when I worked with 'soft' gelatin, after exposure, I let it 10 or 12 hours so dark reaction can take place. After that, I placed plate in an oven for 2 hours at 120°C (248°F). That rises up the yield point for 2 or 3° C.
Advices from Jeff Blyth and Martin Muëller let me guest that I use a too soft gelatin. One of evidence is that even with carefull drying in IPA, color shifted to deep blue after minutes.
Now I use a photographic grade gelatin (220 Bloem) and I have no problem anymore.
That is what I thought. Longer in the hardening bath then the yield point raises. For lower blooms then you want a little extra hardener in the fixer and/or keep in the bath longer.
I think I will go back to Knox as I have those parameter worked out. When I get to the point that I my holograms are so good I want to put the final touch on them I may switch and work out another gelatin's parameters.
"Does anyone know what properties about the gelatin (bloom) and/or the hardener in the fixer or combination thereof that might be causing this."
Generally, it means that the emulsion is too soft. The white fog is basically gelatin precipitating out of the emulsion. If the plate is not sufficently hard before it goes into the water, the water softens and swells the emulsionl. Each alcohol bath removes a little more water but leaves an imbalanced amount of gelatin, ie the emulsion can't support so much uncross-linked gelatin, until finally all the water is driven out and the emulsion can no longer support the gelatin so it "oozes" out and causes a white fog.
By the way, the bloom figure of any commercial gelatin is an approximation. If you're working "on the edge" a small difference in bloom strength may cause effects like the fog. Every time you change a gelatin or composition, you have to tweak a little to get back the results as before.
That's true that since I work with photographic grade gelatin, I 'feel' that gelatin have less whim.
I agree with you that changing a variable give you not other choice than refine your method.
A guy in Belgium Unfortunatly, he stops commercial activity. Market for old photographic process (daguerotype, bichromate gum,...) is too small (like holographic play field).
Did you try photo formulary web site (http://www.photoformulary.com) ?
Make a search for the keyword gelatin.
They sell 250 bloom ($24.95 for 1lb)
