Re: Re: carbon-digest V2 #129

Eric Scott wrote:

> On Thursday, December 22, 2005, at 00:18AM, carbon-digest
> <owner-carbon-digest@spitbite.org> wrote:
>
> The more rapid
>
>> heating with direct double boiler rather than your previous tray
>> may be affecting entrainment of air during dissolution of the
>> gelatin, which would otherwise manage to surface and the bubbles
>> burst before the gelatin is too stiff.
>
>
> I thought about this. With the stove, the gelatin is in a pyrex
> beaker, with the gelatin about 3 inches in depth. I heat the stuff
> slowly, but I noticed that the gelatin at the bottom was *much*
> hotter than that at the top. I would periodically stir it to even
> out the temperature. Perhaps the uneven temperature is causing the
> microbubbles you mention. With the tray method, the gelatin is in a
> very wide/long aluminum container, such that the depth of the gelatin
> is only about 1/4 inch, if that. The stuff heats uniformly.
>

Stirring is another likely source of microbubbles. I see them all the
time when making up Caffenol (coffee developer for silver halide
emulsions), but that's water-thin and the bubbles surface and burst in a
few minutes of standing.

Also, if the pyrex is standing on the bottom of the water bath, it's
being heated directly by conduction from the water vessel, and might get
a lot hotter than it ought to at the bottom where it contacts. If it's
blocked up from the bottom in some manner, that should be less of a
problem, but convection will still preferentially transfer heat into the
bottom compared to the sides (even if the water bath is deeper than the
gelatin in the beaker).

And finally, you clearly have more surface area per volume with the
shallow, tray method, which means any bubbles you do have will have more
opportunities to burst, sooner (before the solution gets too stiff).

-- 
After you use everything you know and can find out to make what you're 
certain is the right decision, and it's too late to change things or 
back out -- you'll find out something that would have reversed the whole 
thing for you, if you'd only known in time.
Donald Qualls, aka The Silent Observer   http://silent1.home.netcom.com
Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.
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