U of S | Mailing List Archive | alt-photo-process-l | Re: bichromate gum

Re: bichromate gum

The speed thing is interesting; Kosar says that am di is faster than sod or pot because of its lower pH (p.52). He also agrees that sod and pot are equivalent in speed (53). He does not go into any of the molar stuff, below, which is why I find that very useful info, Loris. It has been discussed before on the list but you laid it out so nicely :)

But, Kosar shows the speeds of the different dichromates with albumin, gum, and process glue and there are some variations in speed with the colloid used I find quite interesting--albumin being slower to harden with both pot and sod than the other two colloids although with am di it is not (53). This may also explain some differences of opinion as usual--different gums might react differently due to pH or some other factor. And the weird thing is that with gum and process glue, it appears from his chart that sod and am are equal in speed, but with process glue am di is faster. However, in practice when one uses a 5 or 6 minute exposure time, these variations probably don't matter because one always suits practice to what's on hand.

Whether Kosar's info is full of water, holes, or whatnot I cannot judge, but both it and Loris's info confirm my step wedges. But with such variablity possible in gum practice, the only conclusion I come to is all dichromates work just fine.

My initial thought was that sodium dichromate, due to its high solubility, would be SUPER fast, and it is not.

BUT, to me one reason to prefer either sod or am di is that they will not fall out of solution mixed at 15% each (the strength dichromate I use in am di) in cold climates, something I have to watch in MT in the winter as my dimroom gets under 70 often.

I cannot use pot di at 15%, and would have to use it at a lower strength than saturation due to my climate variations (e.g.28 degrees in the morning, 75 in the afternoon, due to elevation of 5000 ft.).

And you cannot use the "dump and pour" method of dichromate mixing with sodium because of its seemingly infinite solubility.

----- Original Message ----- From: "Loris Medici" <mail@loris.medici.name>
To: <alt-photo-process-L@usask.ca>
Sent: Sunday, July 13, 2008 2:28 AM
Subject: RE: bichromate gum

Indeed. There are two forms of NaDi: anhydrous and dihydrate. My
calculation was made according to dihydrate -> since that's what you
"usually" get from the chemical suppliers. Maybe Judy's NaDi was the
anhydrous form, therefore in her experience NaDi was faster than KDi at
the same w/v strength.

Lets re-make the calculation for anhydrous NaDi:

[KDi Strength] * ( [NaDi m.w.] / [KDi m.w] ) = [equiv. NaDi Strength]

10% * ( 262.01 / 294.19 ) = 8.9% (~ 9%)

So, 8.9-9% NaDi is virtually the same as 10% KDi (or 8.5% NH4Di)...

Thank you for the remark Alberto.


12 Temmuz 2008, Cumartesi, 8:15 am tarihinde, Alberto Novo yazmış:
The "reply-to" of Loris' message was set to his personal address instead
The List, so I post my reply again

   sodium dichromate's (NaDi) molar mass is 298.02g
this is for Na2Cr2O7.2H2O.
For the anhydrous form, the M.W. is 262.01. In this case the math changes