I am sitting here thinking about how I wanted to not use fixer or hardener for one of the DCG holograms at the PCG but didn't do it. I wanted to compare the results to the other Holograms. Then it crossed my mind.
As I have seen hardening, post exposure, by both hardener in the fixer and backing without fixer and hardener, what about this technique. Dinesh, you may have enough knowledge on what the hardening actually does to the different areas of the emulsion (exposed and non exposed) to answer this. How about if the plate is baked(hardened) as it is exposed? Would this cause the film to move thus actually destroying the hologram or would the emulsion harden in such a fashion during the exposure to actually cause a greater delta index of refraction between the exposed and unexposed. WOuld it be different for an on axis reflection(fringes perpendicular).If it would destroy the fringes I would still like to know exactly what the hardening process does (physically) to the emulsion.
Thanks as always,
John.
DCG Hardening (Dinesh)
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Dinesh
DCG Hardening (Dinesh)
"How about if the plate is baked(hardened) as it is exposed? Would this cause the film to move thus actually destroying the hologram or would the emulsion harden in such a fashion during the exposure to actually cause a greater delta index of refraction between the exposed and unexposed."
I would imagine it depends on how you bake it. Firstly any heating process will cause physical motion, usually expansion although in this case it'll probably shrink. You have two competing processes: water evaporation and decrease of density of the collagen due to thermal agitation. Whether it shrinks or expands depends on the rate at the heat is applied, or rather the rate of energy transfer (bearing in mind that the room is also a heat sink or source). In either case it's a rate of change of volume and so you have to consider how the volume is changing. Is the area of the face expanding/decreasing faster than the thickness? What is the ratio of the rate of change of thickness dT/dt) to exposure time (E)? If you're shooting at lambda, then you need
dT/dt <= lambda/2E
Irrespective to volume change, you're also causing cross-linking, which is a polymerisation process across hydroxyl groups I believe (Martin, Sergio?). This is certainly temperature dependant, or, more exactly, the rate at which heat is being absorbed. Now again you have two competing processes: actinic polymerisation vs polymerisation due to absorption of phonons in the structure. Which processes dominate, assuming one does, depends on how much light energy you're pumping into it. This is a case of power, not energy.
In both these processes, you also have the effect of dark reaction. The density change is the difference between the cross linked gelatin and the uncrosslinked gelatin. However, the density of the cross linked gelatin is pretty much fixed while that of the uncross linked gelatin depends on water content and other things. The efficency of a reflection hologram is dependant on the peak of the density modulation but the harder the uncrossed gelatin is, the lower the modulation peak. In answer to your earlier question, dark reaction hardens the entire plate overall and so the modulation peak is decreased.
I would imagine it depends on how you bake it. Firstly any heating process will cause physical motion, usually expansion although in this case it'll probably shrink. You have two competing processes: water evaporation and decrease of density of the collagen due to thermal agitation. Whether it shrinks or expands depends on the rate at the heat is applied, or rather the rate of energy transfer (bearing in mind that the room is also a heat sink or source). In either case it's a rate of change of volume and so you have to consider how the volume is changing. Is the area of the face expanding/decreasing faster than the thickness? What is the ratio of the rate of change of thickness dT/dt) to exposure time (E)? If you're shooting at lambda, then you need
dT/dt <= lambda/2E
Irrespective to volume change, you're also causing cross-linking, which is a polymerisation process across hydroxyl groups I believe (Martin, Sergio?). This is certainly temperature dependant, or, more exactly, the rate at which heat is being absorbed. Now again you have two competing processes: actinic polymerisation vs polymerisation due to absorption of phonons in the structure. Which processes dominate, assuming one does, depends on how much light energy you're pumping into it. This is a case of power, not energy.
In both these processes, you also have the effect of dark reaction. The density change is the difference between the cross linked gelatin and the uncrosslinked gelatin. However, the density of the cross linked gelatin is pretty much fixed while that of the uncross linked gelatin depends on water content and other things. The efficency of a reflection hologram is dependant on the peak of the density modulation but the harder the uncrossed gelatin is, the lower the modulation peak. In answer to your earlier question, dark reaction hardens the entire plate overall and so the modulation peak is decreased.
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Dinesh
DCG Hardening (Dinesh)
I'm looking through all my papers on DCG and I'll be sending you a bunch next week.
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Dinesh
DCG Hardening (Dinesh)
"...you're also causing cross-linking, which is a polymerisation process across hydroxyl groups"
The polymerisation is across the carboxylate in the polar section of the gelatin.
The polymerisation is across the carboxylate in the polar section of the gelatin.
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JohnFP
DCG Hardening (Dinesh)
Ok, then, we need to talk. What specifics are involved. Is there a way to heat soas to find the medium between shrinking(losing water) and expanding (heating the water)? This may be a good avenue to explore. If we can find this ballance then the physical motion would be negated, right? How about heating to expand and vacuum to shrink. I remember the reverse explaination that if you vacuum the emulsion it will not expand (because of the vacuum) but constrict because it will lose water. I believe there are many thing not tried yet. I will feel more confident of my proposals once I read the 20 doc I just downloaded today on Collagen and crosslinking.....
Peace!
Peace!
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dcgman
DCG Hardening (Dinesh)
Not sure I understand your reason for thinking that there will be a greater delta-n if the gel is hardened during exposure.
One avenue I have not seen explored on this forum is post-exposure hardening with formaldehyde. Formaldehyde works by predominantly cross-linking the hydroxyl groups of gel molecules, while dichromates work by linking the carboxyll groups.
I've come across a paper in which the Russian authors claim that the sensitivity of DCG can be increased significantly by post-exposure hardening in a formaldehyde bath with a high pH value (~9).The reasoning behind this is that since the hardening is via two distinct chemical routes, one type of hardening (i.e. the "fringes" hardened by the dichromate) cannot be swamped by the other (i.e. the chemical harderner).
I'll see if I can find the references- hope to post these here tomorrow.
One avenue I have not seen explored on this forum is post-exposure hardening with formaldehyde. Formaldehyde works by predominantly cross-linking the hydroxyl groups of gel molecules, while dichromates work by linking the carboxyll groups.
I've come across a paper in which the Russian authors claim that the sensitivity of DCG can be increased significantly by post-exposure hardening in a formaldehyde bath with a high pH value (~9).The reasoning behind this is that since the hardening is via two distinct chemical routes, one type of hardening (i.e. the "fringes" hardened by the dichromate) cannot be swamped by the other (i.e. the chemical harderner).
I'll see if I can find the references- hope to post these here tomorrow.
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Martin
DCG Hardening (Dinesh)
"One avenue I have not seen explored on this forum is post-exposure
hardening with formaldehyde. Formaldehyde works by predominantly
cross-linking the hydroxyl groups of gel molecules, while dichromates work
by linking the carboxyll groups."
Agreed. I several times tried to point to the differences between chrome hardening and aldehyde hardening.
Having read some literature about colloid crosslinking, I gathered the term "hardening" was far from being clear cut. The research field strongly affects the view on "hardening". So if e.g. that view is guided by the making of silver halide emulsions, criteria that are paramount for DCG technology may not count in that case. Well, at least there is one case where the two areas meet: SHSG.
Aldehyde hardening not only implies things like formaldehyde, glutaraldehyde, etc. but also gelatin heating. Upon heating gelatin is said to form in-situ aldehydes which in turn crosslink the colloid.
"I've come across a paper in which the Russian authors claim that the sensitivity of DCG can be increased significantly by post-exposure hardening in a formaldehyde bath with a high pH value (~9).The reasoning behind this is that since the hardening is via two distinct chemical routes, one type of hardening (i.e. the "fringes" hardened by the dichromate) cannot be swamped
by the other (i.e. the chemical harderner)."
That's exactly my take vis-a-vis SHSG (especially with regards to Usanov and the latest paper by Kim/Bjelkhagen/Phillips).
According to Zelikman/Levi there are a couple of theories (e.g. by Sokolov) suggesting a significant difference between the electrochemical properties of gelatin caused by tanning: acidic or basic hardeners. "The first category includes cases in which the tanning agent reacts with the protein to block its basic radicals. This group comprises electronegative semi-colloids or anionic agents, such as vegetable tannins, aldehydes, synthetic tanning agents containing aldehyde groups, sulphonated aromatic compounds, etc. The second group consists of substances which block the electronegative protein groups; i.e., they have basic functions and may be described as cationic. This group includes chromic salts and other inorganic tanning agents." (Zelikman and Levi, Making and coating photographic emulsions, London/New York 1964, p. 269)
In the context of DCG/SHSG I wonder if it might be worth looking into the possibility of specially designing a particular gelatin based on those different hardeners. For example one might add particular amino acids to the gelatin in order to have enhanced response to say aldehyde hardening.
hardening with formaldehyde. Formaldehyde works by predominantly
cross-linking the hydroxyl groups of gel molecules, while dichromates work
by linking the carboxyll groups."
Agreed. I several times tried to point to the differences between chrome hardening and aldehyde hardening.
Having read some literature about colloid crosslinking, I gathered the term "hardening" was far from being clear cut. The research field strongly affects the view on "hardening". So if e.g. that view is guided by the making of silver halide emulsions, criteria that are paramount for DCG technology may not count in that case. Well, at least there is one case where the two areas meet: SHSG.
Aldehyde hardening not only implies things like formaldehyde, glutaraldehyde, etc. but also gelatin heating. Upon heating gelatin is said to form in-situ aldehydes which in turn crosslink the colloid.
"I've come across a paper in which the Russian authors claim that the sensitivity of DCG can be increased significantly by post-exposure hardening in a formaldehyde bath with a high pH value (~9).The reasoning behind this is that since the hardening is via two distinct chemical routes, one type of hardening (i.e. the "fringes" hardened by the dichromate) cannot be swamped
by the other (i.e. the chemical harderner)."
That's exactly my take vis-a-vis SHSG (especially with regards to Usanov and the latest paper by Kim/Bjelkhagen/Phillips).
According to Zelikman/Levi there are a couple of theories (e.g. by Sokolov) suggesting a significant difference between the electrochemical properties of gelatin caused by tanning: acidic or basic hardeners. "The first category includes cases in which the tanning agent reacts with the protein to block its basic radicals. This group comprises electronegative semi-colloids or anionic agents, such as vegetable tannins, aldehydes, synthetic tanning agents containing aldehyde groups, sulphonated aromatic compounds, etc. The second group consists of substances which block the electronegative protein groups; i.e., they have basic functions and may be described as cationic. This group includes chromic salts and other inorganic tanning agents." (Zelikman and Levi, Making and coating photographic emulsions, London/New York 1964, p. 269)
In the context of DCG/SHSG I wonder if it might be worth looking into the possibility of specially designing a particular gelatin based on those different hardeners. For example one might add particular amino acids to the gelatin in order to have enhanced response to say aldehyde hardening.
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JohnFP
DCG Hardening (Dinesh)
My thinking was, and the possible suggestions may not be the avenue toward that goal, is if the emulsion could be hardened in such a was as to harden where the crossling was happening only while exposing, you would get a higher delta in hardened areas. Thus when the emulsion was swollen and dried the two areas would hold water differently thus the difference in index of refraction. THis may take come of the criticallity out of the post drying processing.
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Dinesh
DCG Hardening (Dinesh)
In DCG, it seems to me that to increase the overall hardening by post processing methods would decrease the delta n. If the density of the carboxyl (cross linked) gel remains unaffected but the hardness of the dissolved gelatin were to increase in hardness through hydroxyl hardeining, wouldn't the n of the surrounding gel approach the cross linked gel and decrease delta n?
One thought occurs to me: The Kramer-Kronig relations tell us how n changes as pho (density) changes. However, if you arrived at a particular density through hardening via the carboxyl groups would it affect the n differently than if the hardening were to occur through the hydroxyl group. I'm vaguely thinking of the molecular polarity of the amino acids and its influence in the carboxyl hardening method which does not occur in the hydroxyl path. Another thought is that if the hydroxyl bonds could be broken as a post-processing method without affecting the cross linked gel hardness, would not the delta n increase?
One thought occurs to me: The Kramer-Kronig relations tell us how n changes as pho (density) changes. However, if you arrived at a particular density through hardening via the carboxyl groups would it affect the n differently than if the hardening were to occur through the hydroxyl group. I'm vaguely thinking of the molecular polarity of the amino acids and its influence in the carboxyl hardening method which does not occur in the hydroxyl path. Another thought is that if the hydroxyl bonds could be broken as a post-processing method without affecting the cross linked gel hardness, would not the delta n increase?