Dichromate or Chromate?

Hi All,

While reading various literature related to alternative processes
I found that sometimes authors advice add ammonia to the
potassium bichromate solution till the color of mix turn to yellow (like straw).
This advice relates to bath sensitizing (sensitizing by immersion) and
to Carbon, Oil printing and other chromate-gelatin based processes.
As I know this is chemical reaction that transform potassium bichromate
to the potassium chromate.
I've tried to find any information related to using potassium chromate in alternative processes and could not.
Only one thing I've found on holography forum: they think that chromate less light sensitive that dichromate.

Has somebody any information about using of potassum chromate insted of dichromate?

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Thank you

I don't remember having come across the references you mention concerning carbon, oilprints, etct.

However, I have been making casein di(or bi)chromate prints for years, and I always dissolve the powdered casein in household ammonia. When I add the dichromate, it turns yellow, and I was informed before the this means it has turned to (mono)chromate. The odd thing is that my casein emulsions print considerably faster (1 1/2 - 2 stops) than my gum emulsions. I never bothered for an explanation of this, but I have observed it consistently (though I print only rarely with gum).

Yes you are right with the Casein print. There is the same reaction in this procees.
What I'm thinking about is why I need to use dichromate and add ammonia, instead of using chromate salt.

Moreover I found that Potassium chromate is less dangerous than dichromate:

Potassium Dichromate
Health Rating: 4 - Extreme (Cancer Causing)
Flammability Rating: 0 - None
Reactivity Rating: 3 - Severe (Oxidizer)
Contact Rating: 4 - Extreme (Life)

Potassium Chromate
Health Rating: 3 - Severe (Cancer Causing)
Flammability Rating: 0 - None
Reactivity Rating: 3 - Severe (Oxidizer)
Contact Rating: 3 - Severe (Corrosive)

This is the Carbon process description: http://www.edinphoto.org.uk/1_early/1_early_photography_-_processes_-_carbon_print.htm

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The sensitising solution is made by dissolving 1oz potassium bichromate and 5 drops liquid ammonia (.880) in 20 oz distilled water.
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unrealalex said:

Hi All,

While reading various literature related to alternative processes
I found that sometimes authors advice add ammonia to the
potassium bichromate solution till the color of mix turn to yellow (like straw).
This advice relates to bath sensitizing (sensitizing by immersion) and
to Carbon, Oil printing and other chromate-gelatin based processes.
As I know this is chemical reaction that transform potassium bichromate
to the potassium chromate.
I've tried to find any information related to using potassium chromate in alternative processes and could not.
Only one thing I've found on holography forum: they think that chromate less light sensitive that dichromate.

Has somebody any information about using of potassum chromate insted of dichromate?

---
Thank you

Click to expand...

In the days before refrigeration was common many carbon printers routinely added enough ammonia to the potassium dichromate sensiteizer to turn it yellow, and into the chromate. Carbon tissue sensitzed in a chromate has more contrast but slower printing speed than one sensitized in a dichromate solution of the same strength. More importantly, it will last longer, up to several days if stored in good condition. Carbon tissue sensitized in dichromate is only good for a few hours before it begins to fog, and will be useless in most climates 24 hours after sensitizing.

When I first began printing with the carbon process I experimented with chromate sensitizing, but it is much more practical today to just place the sensitized tissue in a zip-loc bag and store it in the freezer, where it will be preserved for weeks or even months.

I see you are in St. Petersburg. Just wonder if you happen to know Vladimir Nikitin. He is a photographer from St. Petersburg who has done quite a bit of work on photo history. I met him once in St. Petesburg, and he later spent a year as a guest instructor at my university in the US.

Sandy King