I would like to simplify the discussion of gum solutions by omitting water from all discussions. After all water is only incidental in helping spread the sensitizer and making the solution. Therefore use of saturated or diluted dichromate is also of no grat importance. Once the paper is coated and dried, water has no bearing on composiion, it has evaporated. I would like to propose to express things basis the actual sensitive gum layer concentration. That is a ratio of dry pigment to dry gum and ratio of dry dichromate to dry gum. These are concentrations of both components in solid gum solution.
Let's take the water out. Is is not present in the gum layer when you are exposing it to UV light.
Hereis what I have for your and mine ratios:
|14 baume gum, 25%
| dry gum at 0.25%, g
|dry pigment, g
|additional gum, g(paint)
|Cr2 from sol, grams
SO it seems our dichromate concentrations are very similar, but you have about twice as much pigment by this calculation. To roughly check and crosscalibrate your method, I took a lamp black watercolour paint and diluted it with 14 baume gum. The solution consisted of 3 grams of lamp black paint in 10 cc of gum. I did the smudge test on this and my 2.5% gum stock. I have to say from visual observation that my stock is somewhat (but not much) darker then what I have from the watercolour paint, but the calculations above show that youI should have more pigment (I can send the jpeg of the smudge test). I am inclined to think that the paint is less concentrated then 25% or the pigment is less black.
ANother observation is that with so much paint one can bring a substantial amount of extenders, and other substances used in paints and that can alter properties of gum solution.
I would suggest buying carbon black from Daniel SMith. It is very inexpensice (a few $ per lb) and allows for a controlled experimentation.
The discussion is great though.
> Date: Sun, 2 Dec 2007 12:35:02 -0800
> From: firstname.lastname@example.org
> Subject: Re: Bleach-development with gum recipe
> To: email@example.com
> Thanks! That's helpful. I was thinking you were working with a
> dilute dichromate and that maybe my mottling had something to do with
> using saturated ammonium dichromate, but obviously that's not it,
> since you're using the same.
> So many differences. First the pigment: Even if I had a way to
> weigh my paint, comparing our actual amounts of pigment would be
> difficult and could only be a ballpark estimate, since there's no
> way to tell how much actual pigment is in a given amount of paint.
> But using Bruce MacEvoy's figure of 20-30% pigment/vehicle for carbon
> black, I'll use 25% as a middle figure to ensure a conservative
> estimate, although M. Graham tends to be on the high end as far as
> pigmentation of their paints. Then considering how much of the tube
> I squeezed out into around 10 cc of gum --- I'd guess 1/5 of the
> tube, or 3 grams of paint. That's a LOT of lamp black, but following
> Marek's specification of a "highly pigmented" mix, I wanted to really
> load it up. Okay, 25% of 3 cc= .75 grams of pigment in 10 ml gum,
> waving aside the little difficulty about the density of gum and all
> that, gives 7.5%. It's not exact, but probably not all that far
> off, and I think I'm satisfied that it corroborates my evaluation of
> this mix as heavily pigmented. That was the first mix; the second
> one I derived by adding around 5 cc gum to the first mix.
> Oh, but then you add water, which dilutes the pigment as well; I
> don't add any water to the mix. So a more comparable figure than %
> pigment in pigment mix would be % pigment in total coating mix.
> Here's how I figure it, and I hope someone will check my figures,
> because while I'm pretty good at statistics, I've never been great at
> simple arithmetic: For Marek: 2.5 g in 100cc = .025 g per cc. So
> 12 cc's contain .3 g pigment. Total mix = 12 gum/pigment plus 12
> water + 6 dichromate solution= 30 cc total mix (never mind that the
> sum of the liquids might not add to 30 for a number of reasons; we're
> just getting a number to compare, here) so .3 g pigment in 30 cc
> total solution = 1% pigment to total coating mix.
> Then mine: the pigment mix contains an estimated .075 g pigment per
> cc gum. my coating mix consists of 2.5cc of pigment mix, so 2.5x .
> 075 = .1875 g pigment. Total mix = 2.5 cc gum/pigment + 2.5 cc
> ammonium dichromate soluton = 5 cc total mix. .1875 g pigment in 5
> cc total = 3.75% pigment to total coating mix.
> That was a lot to go through to convince myself that I'm using more
> pigment as a proportion of mix than you are, but I needed to be
> convinced for sure. So I can cut back on the pigment by at least
> half and still have what you would call a "highly pigmented" mix that
> can be brushed out thin. Half as much pigment will give a mix that
> will brush out thin; I prefer to reduce the amount of pigment rather
> than adding liquid.
> I don't like Fabriano paper, and recently used up the last of my
> sample of Fabriano Artistico extra-white running some test prints
> just to get rid of it, so our papers will have to be an uncontrolled
> variable, also sizes, as I like glyoxal.
> We've got a big storm blowing in this afternoon, and this is helping
> distract me from the sound of the wind. Unfortunately, I put the
> car in the garage to protect it from flying branches etc, and now
> it's really hard to get to my darkroom, which opens off the side of
> the garage. I prefer to leave the car outside and use the garage
> as a lobby-area for the darkroom (speaking of Judy's characterization
> of our wide-open spaces out here in the west).
> Report to follow, unless the power goes out. Over,
> On Dec 2, 2007, at 11:14 AM, Marek Matusz wrote:
> > Here is my recipe for sensitizer for bleach develop:
> > Gum pigment solution:
> > 100 cc of 14 Baume commercial gum 2.5 grams of Daniel Smith carbon
> > black powder, 4 drops of jet dry. Jet dry is an automatic
> > dishwasher rinse aid. I found this detergent very helpful in
> > dispersing carbon black powder, which is difficult to wet. Jet dry
> > is a miracle detergent for me to help with coating, reduce bubbles,
> > etc.
> > All that mixed in a mixer for a few minutes. Last batch (0.5
> > liters) was made several months ago. I just shake it before using.
> > I call it a 2.5% carbon solution, although in reality it is higher,
> > since gum density is less then 1.
> > Preparing sensitizer:
> > 12 cc of gum pigment solution, 12 cc of water, 6cc of 30% ammonium
> > dichromate. This gives me the right consistency for my coating
> > method. It is also optimum in terms of having enough water to wet
> > the paper and keep it moist long enough to smooth the coat, but not
> > to keep it too wet. My paper has been Fabriano Artistico white and
> > extra white since my first tricolor gum a few years back. When
> > sized both papers act in the same way. I suppose I am so addicted
> > to that paper that if they ever stop making it I'll have to quit
> > printing.
> > Sieze is two coats of 3.5% gelatin hardened with glutaraldehyde or
> > recently with dichromates. The examples on the web site were made
> > on paper hardened with dichromates.
> > Exposure is 3 minutes under UV BL light bank. My typical gum
> > exposure is 45s to 1 minute.
> > Development:
> > Soak briefly (1-2 minutes) in water to remove dichromates. Soak in
> > bleach solution (20ml of bleach/liter) for 1 to 5 minutes, finish
> > developing in water. 30 minutes to 2 hours are my typical times for
> > water development. If development is very slow go back to bleach
> > for a few minutes. I change or replenish bleach every 2 or 3 prints.
> > Have fun printing
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