Cross Linking

Tony · Post by **Tony** » Fri Dec 03, 2010 12:28 pm

I couldn't find this in the old forum but I know it was discussed.
Does AmDi crosslink when in a warm gel state?
Thanks!!

Joe Farina · Post by **Joe Farina** » Fri Dec 03, 2010 1:35 pm

Yes, heat or warm storage conditions will increase the dark reaction dramatically, I think.

Tony · Post by **Tony** » Fri Dec 03, 2010 2:12 pm

Thanks Joe,
I know Dave had said he mixes at 70C. I tend to stay in the 40C range. Do you think that makes a difference?
Obviously Dave makes greats DCGs so it can't be that bad.

Lastly, Rallison says that high humidity and high temps causes faster dark reaction. I wonder how that works out when it is in solution?

Joe Farina · Post by **Joe Farina** » Fri Dec 03, 2010 2:38 pm

I doubt that the different temperatures during mixing, as you mentioned, would make much difference. There might be some extra crosslinking at higher temperatures, but as long as you're consistent with temperatures, I don't think it would be anything to be concerned about. Also, since the heating during mixing occurs for a short time period, it may not have time to do much crosslinking. It might even be beneficial to have some extra hardening occuring in the beginning. My DCG always worked better after ageing a while in the fridge.

Usually in the literature they recommend the lowest possible temperatures for melting gelatin, though, and always with a double-boiler method. I think Rallison occasionally had some kind of "pitting" problems, which he thought might be due to over-heating. He described these as some kind of "pits" or tiny dots in the emulsion, but I've never seen them myself.

a_k · Post by **a_k** » Fri Dec 03, 2010 3:50 pm

The longer gelatin is heated, the more it's bloom strength and its ability to absorb water decrease. But even 70°C, as long as it is kept for minutes and not hours, wouldn't make a dramatic change i think.

Concerning the increased rate of dark reaction, as a rule of thumb an increase of 10°C doubles the speed of chemical reactions. That would mean that 2 minutes at 70°C correspond to the same level of hardening as 1 hour at room temp.

Tony · Post by **Tony** » Sat Dec 04, 2010 11:56 am

Wow thanks, that is very instructive.
Sometimes it is hard to seperate myth from fact.
There are so many variables to consider so little time.

I also see tiny pits in the emulsion which I thought was due to dust specks. I do not allow my film to get over 42C but there could be localize heating due to the heater. I place my beaker in a water container which on top of my stirer/heater. It could be that there ares spots where the pyrex in closest to the heater is hotter. On the other hand the stirer is always moving so maybe not

Joe Farina · Post by **Joe Farina** » Sat Dec 04, 2010 12:56 pm

These are a couple pages from the notes for a class given by Richard Rallison, on the subject of film preparation and "pits."

Joe Farina · Post by **Joe Farina** » Sat Dec 04, 2010 1:00 pm

Tony, I had some trouble matching the resolution of the scanned images to what is acceptable for attachments. I will email those two pages to you.

dannybee · Post by **dannybee** » Sat Dec 04, 2010 1:21 pm

Joe Farina wrote:These are a couple pages from the notes for a class given by Richard Rallison, on the subject of film preparation and "pits."

can you repost this? tried to print it but way to small

Joe Farina · Post by **Joe Farina** » Sat Dec 04, 2010 2:31 pm

I emailed them to you Danny. When I did a scan at 75dpi, I got a message saying it was too many pixels. That was already pretty low resolution, so I did one at 50dpi, which worked, but it's not readable. Sorry I'm not able to debug this process right now.