Something bothering me about time spent doing mold, and time required in the fridge to set gelatin. What if I spent too much time stacking molds in the box, then whole box put in the fridge. What if gelatin don't like too rapid cooling? My fridge is empty now and cooling very rapidly.
If gelatin has 'strands' (I don't know exactly what is happening here), and such strands need to be 'interwoven' somehow in order to set, and become gummy. Last formula has very low amount of gelatin, just 5%, so maybe water re-dissolving it even at low temperature? Or, maybe 5C is too low temperature for gelatin? Or maybe opposite happening - viscosity at 5C is too high and thus slowing down everything?
Very brief explanation on wikipedia about what is the "Bloom test":
"Bloom is a test to measure the strength of a gel or gelatin. The test was originally developed and patented in 1925 by O. T. Bloom. The test determines the weight (in grams) needed by a probe (normally with a diameter of 0.5 inch) to deflect the surface of the gel 4 mm without breaking it. The result is expressed in Bloom (grades). It is usually between 30 and 300 Bloom. This method is most often used on soft gels. To perform the Bloom test on gelatin, a
6.67% gelatin solution is kept
for 17-18 hours at 10°C prior to being tested."
(important parameters in bold and underlined)
So, 17-18 hours at 10C for gelatin of 6.67% (100:6.67:X). If Bloom test is for pressing, or mechanical stress, then pulling mold apart cause stress as well, where gelatin at such short time did not set properly: only 2 hours, planed even shorter - 30 minutes, but my fridge was full of ice at this time, so I decided to wait more.
Maybe it is good to change setting on the fridge from 5C to 10C at one part of it (I have thermometer probe which can protrude box with plates, so can control whether it reaches such and such temperature)?
Also, I am not sure at all in my gelatin I use (Podravka's Dolcela). On the net, there is some information about food grade gelatin, but that figure may be somewhat inconclusive:
"Gelatin used in food usually runs from 125 Bloom to 250 Bloom. There are several different grades of sheet gelatin. The most popular are Silver grade (160 Bloom) and Gold grade (190-220 Bloom). Typically the higher the Bloom, the more you can expect to pay."
I have Dr.Oetker "Gold extra", and I hope this means "Gold grade" (190-220 Bloom). The reason what I did not use it in past time is observation that both; powdered form and leafs give me somewhat less clear mixture, but that may be completely subjective. And, that statement about the price; both gelatins are almost the same price - not related to the quality of the gelatin, but rather on market prices (First one is 'domestic' product, another one is imported).
---
Aside all that problems, another problems are with understanding Pantelic's document:
"The dependence of diffraction efficiency on the concentration of Rhodamine 6G is shown in Fig.3. As the concentration of Rhodamine 6G increases, the diffraction efficiency increases; it reaches a maximum when the dye concentration is 7.5 mg/l. After the maximum concentration is attained, the diffraction efficiency decreases because Rhodamine 6g precipitates at higher concentrations in the presence of the dichromate ion."
- Some comparisons of DCG and dyed DCG...
It looks too 'sharp' to me, but that is because scale is from 20-32% diffraction efficiency, not gain in the sensitivity. Another graph showing absorption curve, where it looks like DCG + Rh6G has little bit over 100% more absorption, and if this means doubling gain, then it is worth messing with. But in text, there is value of sensitivity increasing from 215 mJ/cm^2 to 140 mJ/cm^2, which is more in agreement with graph above, or increasing of the sensitivity of 60% (still not bad).
The absorption graph:
- Fig. 3, from Pantelic's document.
Also, there is problem how to calculate concentration of the Rh6G if I want to increase gelatin content (for example from 100:5:0.5 to 100:10:1)? I made stock solution of 75 mg/l (the most difficult part is to weight such small amount, and I had limit of hundreds of milligrams - impossible to measure tens of milligrams). This solution then is diluted 1/10 to the mixing solution, i.e. 90 ml of water, 10 ml of concentrated solution.
I will try to make mold coating again, and this time will try to increase concentration. But, this time will leave it longer in the fridge (maybe 12 or better 24 hours) and frequently checking whether it set or not.
Best--
m--
Something bothering me about time spent doing mold, and time required in the fridge to set gelatin. What if I spent too much time stacking molds in the box, then whole box put in the fridge. What if gelatin don't like too rapid cooling? My fridge is empty now and cooling very rapidly.
If gelatin has 'strands' (I don't know exactly what is happening here), and such strands need to be 'interwoven' somehow in order to set, and become gummy. Last formula has very low amount of gelatin, just 5%, so maybe water re-dissolving it even at low temperature? Or, maybe 5C is too low temperature for gelatin? Or maybe opposite happening - viscosity at 5C is too high and thus slowing down everything?
Very brief explanation on wikipedia about what is the "Bloom test":
"Bloom is a test to measure the strength of a gel or gelatin. The test was originally developed and patented in 1925 by O. T. Bloom. The test determines the weight (in grams) needed by a probe (normally with a diameter of 0.5 inch) to deflect the surface of the gel 4 mm without breaking it. The result is expressed in Bloom (grades). It is usually between 30 and 300 Bloom. This method is most often used on soft gels. To perform the Bloom test on gelatin, a [u][b]6.67% gelatin[/b][/u] solution is kept[u][b] for 17-18 hours at 10°C[/b][/u] prior to being tested."
(important parameters in bold and underlined)
So, 17-18 hours at 10C for gelatin of 6.67% (100:6.67:X). If Bloom test is for pressing, or mechanical stress, then pulling mold apart cause stress as well, where gelatin at such short time did not set properly: only 2 hours, planed even shorter - 30 minutes, but my fridge was full of ice at this time, so I decided to wait more.
Maybe it is good to change setting on the fridge from 5C to 10C at one part of it (I have thermometer probe which can protrude box with plates, so can control whether it reaches such and such temperature)?
Also, I am not sure at all in my gelatin I use (Podravka's Dolcela). On the net, there is some information about food grade gelatin, but that figure may be somewhat inconclusive:
"Gelatin used in food usually runs from 125 Bloom to 250 Bloom. There are several different grades of sheet gelatin. The most popular are Silver grade (160 Bloom) and Gold grade (190-220 Bloom). Typically the higher the Bloom, the more you can expect to pay."
I have Dr.Oetker "Gold extra", and I hope this means "Gold grade" (190-220 Bloom). The reason what I did not use it in past time is observation that both; powdered form and leafs give me somewhat less clear mixture, but that may be completely subjective. And, that statement about the price; both gelatins are almost the same price - not related to the quality of the gelatin, but rather on market prices (First one is 'domestic' product, another one is imported).
---
Aside all that problems, another problems are with understanding Pantelic's document:
"The dependence of diffraction efficiency on the concentration of Rhodamine 6G is shown in Fig.3. As the concentration of Rhodamine 6G increases, the diffraction efficiency increases; it reaches a maximum when the dye concentration is 7.5 mg/l. After the maximum concentration is attained, the diffraction efficiency decreases because Rhodamine 6g precipitates at higher concentrations in the presence of the dichromate ion."
[attachment=0]Figure 3.jpg[/attachment]
It looks too 'sharp' to me, but that is because scale is from 20-32% diffraction efficiency, not gain in the sensitivity. Another graph showing absorption curve, where it looks like DCG + Rh6G has little bit over 100% more absorption, and if this means doubling gain, then it is worth messing with. But in text, there is value of sensitivity increasing from 215 mJ/cm^2 to 140 mJ/cm^2, which is more in agreement with graph above, or increasing of the sensitivity of 60% (still not bad).
The absorption graph:
[attachment=1]Figure 1.jpg[/attachment]
Also, there is problem how to calculate concentration of the Rh6G if I want to increase gelatin content (for example from 100:5:0.5 to 100:10:1)? I made stock solution of 75 mg/l (the most difficult part is to weight such small amount, and I had limit of hundreds of milligrams - impossible to measure tens of milligrams). This solution then is diluted 1/10 to the mixing solution, i.e. 90 ml of water, 10 ml of concentrated solution.
I will try to make mold coating again, and this time will try to increase concentration. But, this time will leave it longer in the fridge (maybe 12 or better 24 hours) and frequently checking whether it set or not.
Best--
m--