Melting and mating

Witho,

We were working without air conditioning during Santa Fe's heat wave last
summer. It was a study group and a class at the local college for several of
the participants.

I put a few quarts of rubbing alcohol in the freezer and using that dropped
down the sensitizer temp to about 65 Deg F. The tap water temp was running
in the high 70's and maybe even as high as 80. The water travels overhead in
a 2000 sq foot industrial building that was hot so I we never could get cool
water.

 Even so in a room that was 95 deg F., even if we started with cool
sensitizer, the sensitizer would warm up and then the tissue would start to
melt. You could see the pigment starting to stain the coating solution in
the white tray as we brushed. That was a sign to stop.

Another problem due to heat was mating was difficult and was nearly
impossible. So when we were mating we put a tray of ice water over the
mating pair. This cooled them down and pressed them together and made for a
much more successful mate.

>From my 1920's book on gelatin I surmised this:

Gelatin swells better and faster in cold water. This is counter intuitive
but readily observed in practice.

When gelatin swells osmotic pressure builds up -- positive in the direction
the water is moving and negative where it is coming from.

Swelling ceases when the water temp nears or reaches the set point of the
gelatin.

What makes the gel stick is a sort of "vacuum" created at the point where
the gelatin on the tissue meets the support. The outer surface of the gel is
wet and the support paper is wet and as the water migrates up into the
deeper layer of gelatin moving towards the tissue support a "vacuum" is
created at the gelatin and final support layer, thus making the gelatin
stick.

My gelatin has a set temp in the mid 90's so you can see there is a problem
when the room is that hot. What I think happened was that the swelling
ceased due to the heat and there was no "vacuum" sticking it down. Indeed
before we used the ice water tray, images just slid off the paper (Yupo in
this case.)

We had god success the Saturday before and the only difference was the
extreme heat in the lab so the hot day was the tip off.

--Dick

-----Original Message-----
From: owner-carbon@spitbite.org [mailto:owner-carbon@spitbite.org] On Behalf
Of Witho Worms
Sent: Friday, December 22, 2006 5:21 PM
To: carbon@spitbite.org
Subject: RE: [carbon] acetone

Dick,

More than a month ago I wrote to the list with a question about a 'melting'
top layer of tissue while sensitising.

(We just stopped when the pigment started to come off the tissue.)

Can you tell me more about this.

Witho

-----Oorspronkelijk bericht-----
Van: owner-carbon@spitbite.org [mailto:owner-carbon@spitbite.org] Namens
Richard Sullivan
Verzonden: vrijdag 22 december 2006 19:52
Aan: carbon@spitbite.org
Onderwerp: RE: [carbon] acetone

  Vaughn,
  
  With a flooded brush method I believe there is a time/temp parameter. Last
summer when the print room was in the mid 90's (that's Deg F, not Deg C.!)
there was virtually no time/temp control. We just stopped when the pigment
started to come off the tissue.
  
  Even at that I've seen little change in the prints using the same
sensitizer % that was used in the summer.
  
  If you are shooting for relief, this may be a factor however.
  
  We will be producing a high relief tissue in the future and Sandy's data
will come in handy. Our first tissue to market will be standard type of
tissue since I think we can all agree the printing parameters are a bit
simpler for the novice. Sandy alluded to the edgy parameters on high relief
in a recent message.
  
  High relief is an issue of personal taste. I think it is somewhat like
brush marks on a pt print. I think it is an interesting and novel idea for
portfolio prints, but once under glass in a frame, it is not that visible. I
am sort of neutral on the subject. I think it would be nice for some prints
and immaterial for others.
  
  --Dick
  
  
  
  -----Original Message-----
From: owner-carbon@spitbite.org [mailto:owner-carbon@spitbite.org] On Behalf
Of Vaughn Hutchins
Sent: Friday, December 22, 2006 11:04 AM
To: carbon@spitbite.org
Subject: Re: [carbon] acetone
  
  Sandy and Howard,
  
  I noticed that you use half the sensitizer that I do.
  
  What changes in the print would I see (contrast/exposure), if everything
  else is kept constant and I used 15ml of sensitizer rather than 21ml for
an
  8x10?
  
  Would I pick up some deeper penetration of the UV due to less Ammonium
  dichromate due to less volume used (thus less UV blocked by the yellow of
  the Am. Dichromate)?
  
  Many thanks,
  
  Vaughn
  
  PS Dick's latest post arrived as I was composing my questions. The
method
  "perfected" by Howard seems to suggest that the amount is not critical --
  but perhaps I am wrong on this if Howard times his sensitizing and
controls
  amount that way. Or perhaps the gelatin absorbs as much as it can and
that
  gives consisantcy. Or is it strickly a matter of concentration -- not
  volume?
  
  Perhaps the larger amount of sensitizer I use helps reduce streaking?
  
  So many variables!!!!!
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Received on Fri Dec 22 18:45:23 2006