Wayde Allen (wallen@boulder.nist.gov)
Fri, 04 Jun 1999 09:17:59 -0600 (MDT)
On Fri, 4 Jun 1999, Paul Lehman wrote:
>
> > One interesting note from the Ward, Courts book
> > (page 509) is that "Bloom
> > strength is approximately proportional to the square
> > of the solution
> > concentration."
>
> On the surface this seems contradictory. If a 7% concentration of one
> gelatin gives a bloom rating of X, and a 7% concentration of another
> gelatin gives a bloom rating of Y, how can the bloom rating be
> proportional to the concentration if the solutions were prepared at the
> same concentration?
No I think you missed the point. What the quote I gave is saying is
nothing more than that the bloom test is sensitive to the gelatin
concentration - "approximately proportional to the square of the
concentration". This means you need to be careful if you want to get
repeatable measurements. The continuation of the quote (I think I gave
this originally) is:
"Thus it is necessary, in routine Bloom determinations, to weigh to and
accuracy of +/- 0.01g. The addition of 105 ml of water to the gelatin
samples inconveniently accomplished with an automatic pipette. This
should deliver 105 +/- 0.1 ml."
For example, let's assume you have a 175 bloom sample. If we consider the
sensitivity relation given above you'd get:
bloom = A * (concentration)^2
or 175 = A * (7.5/105)^2
from which we can get A = 34300 (This is the constant of
proportionality.).
Now say we want to repeat the measurement, but make a 0.02 g error in
measuring the gelatin. Now we have
bloom = 34300 * (7.52/105)^2 = 175.9 or 176 bloom.
If we are off by as much as 0.1 g then we'd get
bloom = 34300 * (7.6/105)^2 = 179.7 or 180 bloom!
In other words, you could measure two samples from the same batch of
gelatin and get widely different bloom ratings if you are a little bit
careless.
> One possibility is that bloom rating may actually be a function of the
> molarity of the solution rather than the concentration. In other words,
> a 7% concentration (by weight) of one gelatin may contain more or less
> the number of molecules of protein than another gelatin with a
> different bloom rating (this assumes that non-protein impurities that
> may be present do not affect the bloom rating) (or could the bloom
> rating be a function of impurity content?).
Not as I understand it. This is the sensitivity of the test to the test
conditions.
> (We may also have to draw a distinction between gelatin being "hard"
> when in the gel state, and being "hard" in the dry state, meaning
> requiring a high temperature to cause disolution).
I agree that there is a problem here with the definition of "hard". For
carbon printing I think that it is probably a bit more descriptive to say
that the exposure changes the solubility of the gelatin rather than its
"hardness". That is not to say that the film doesn't actually get harder,
but I think we are more interested in the water solubility. Perhaps the
direct carbon processes using sawdust slurry for development makes use of
the actual hardening?
> If I have read the responses correctly, it appears that this issue
<bloom number relationship to dichromate sensitivity>
> has not been specificly addressed in a controlled study (although it may
> be in the deep literature somewhere).
I think the correct answer is that it doesn't apply. The bloom test is
not specifically designed to measure dichromate sensitivity. It is a gel
strength test, nothing more.
> However, to be able to classify
> gelatins by there mechanical hardness there must be a chemical
> difference as well (again, assuming that impurities do not play a
> role). The hardness of a gel is a function of water content, the
> molecular structure of the protein to hold the water, and the probable
> propencity for hydrogen bonding. If this is true, the bloom rating must
> be indicating a molecular difference in the gelatin protein, either
> longer or shorter chain lengths, the shape of the protein (coiled or
> straight), or in the number of functional groups (amines, hydroxyls,
> etc) present or exposed on the surface of the protein molecule.
>
> So, my guess is that dichromate sensitivity may be a function of Bloom
> rating.
That is the tricky part of your question. There may be "some"
correlation.
> Those factors that may influence bloom rating should also
> influence dichromate cross-linking of the gelatin (gelatine size, shape
> and availability of functional groups). However, the degree of
> sensitivity corrilation to the bloom rating is the final question.
One could pursue this, but I'm not certain that it makes sense. I could
also try to correlate the outdoor air temperature to how slick the roads
might be too, but as you've noted there are other factors at play. I
think it simply makes more sense to simply measure the dichromate
sensitivity of a given gelatin rather than to try and infer it from a
measurement not designed to provide this information.
- Wayde
(wallen@boulder.nist.gov)
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