Re: warning Re: gum mixes
On Fri, 4 Jan 2008, sam wang wrote:
... I have yet to see dichromate in powder form - what I use is in crystal form and with the same respect I have for any other photo chemicals, I don't find the danger of inhaling it unless I stick my nose into the crystal and clearly I don't do that.OK Sam, you got me there. I suppose what I've been using is technically "crystal form" -- " or BEGINS that way. But IMO you make a distinction without a difference.
Technically I suppose there is a difference at the outset, but (& you have cost me half an hour of my remaining time on earth), there are so few places to buy chemicals these days, I could find very few listings (Photog. Formulary, Kremer, Dan'l Smith) NONE of which specified whether powder or crystal or flake or grain or crumble.. They just listed the name and price.
So I laid hand on "Photography: Theory & Practice," for some reason both first & 2nd edition (I probably forgot I had the first & bought the 2nd, signed by a Jack Rubin -- anybody know him? ) Anyway, I found in the 2nd on page 414, section 637 under "Chromates and Bichromates" that "Potassium bichromate...occurs in large orange crystals." and "Ammonium bichromate is in smaller crystals and distinctly redder in colour..." etc.
But what happens to crystals when they sit in a jar and jostle each other at night, or jockey for space when you pick up and pour, or just age until their scales fall into dust? So I picked up the only bottle of actual dichromate I could locate -- probably originally contained a kilo, must be about 90 years old (the chem department cleaned house & gave it to me, I rescued it from the dumpster). The label says "AMMONIUM BICHROMATE photo *granular* POISON."
It is still for the most part in the form of tiny (crystals to you, granules to them), which I would -- uncorrected by you -- have called a powder. It's fairly lumped together, that is, when I turn the jar upside down, nothing leaves the mass. But are you going to claim that when I pry with a knife to loosen the grain to make a pinch or a gram or whatever I need, that nothing sheds, no powder forms? In fact when I look at it under a loupe I already see tiny specks among the granules -- that is, I daresay "powder."
Do your "crystals" shed less than mine? Make less "powder'? Or even tinier "crystals" that could float in air? And would you want your students to breathe them? Even a few grains? (How many grains of anthrax would you recommend? You yourself are clearly Mighty Man whom no crystal can offend, but we are mere mortals.)
So OK, dichromate is less deadly than anthrax, in fact workers in the 19th century used it barehanded... (and got poisonous lesions that crippled them... rarely finding the cause).
So not having seen the matter mentioned, as it usually is not, I used the term "powder" loosely. The dictionary (Websters unabridged) says powder is a "substance composed of fine particles as a dry pulverized earth ... or ... a powdered preparation; a product in the form of discrete, usually fine particles", which covers the dichromate I've seen & no doubt yours as well.
I don't want to see my students handle it in any way every time they make a print... nor myself either, or for any of us to do anything we don't have to do that could make more powder from "crystals." Which is why I mix up one concentated solution at home for myself (wearing a mask) and use it (diluting) as long as it lasts, and have students mix up a concentrated solution at the beginning of the semester with me watching like a hawk and barking orders and warnings -- so they've done it under supervision.
As for using more dichromate than necessary, you seem to imply that I do. Of course I am aware that di concentraton affects contrast, and dilute accordingly at use. I mix a saturated solution because (best information is) it keeps better that way -- and I've certainly never found it failing to keep... There's another reason for us city dwellers -- same amount of am di solution has a smaller "footprint" when saturated.... and shelf space is a precious commodity in a city studio.
I prefer to use about 1/8 of dichromate in dry form for gum as compared to using saturated solutions. Didn't we go over this many times in the past? I can't understand why one would use more dichromate than necessary to do the job, except because the folks who wrote the literature way back when said to do so.
They obviously did not see the danger of dumping more dichromate down the drain way back then.
If they had drains.
As noted... also the amount of liquid, regardless of "concentration" is another important variable.The degree of dichromate concentration does affect the contrast.
This is no longer a problem. With digital negatives one can easily accommodate the change. Just do a test with the step tablet. Speed does not change very much, except in extreme cases.True, I daresay, but I also I have many old film negatives I plan to reprint.
It would be a better world if we all dump less dichromate down the drain, and one way of doing that is to use smaller amount of dichromate instead of saturated solutions. Using dry crystals is the easier way to accomplish that.As noted, I am not using saturated solution, but quite diluted, & doubt
that I put more actual dichromate "down the drain" than you do, maybe less!!!
So there !
In any event my experience is as noted:
and... it's anyway far better to make a strong mix in liquid and dilute as
if necessary -- quicker, safer and keeps indefinitely in solution. plus wherever it is is always visible. (The fine dry powder is not -- and it does enter the air.)