Re: Bleach-development with gum recipe
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: