Reusing Potassium Dichromate Sensitizer for Carbon + Brush Sensitizing?

[holmburgers]

Hi all,

This is a 2-part question. Respond to whichever you prefer

1st, I had a solution of 3.5% potassium-dichromate mixed with distilled water. I opened it up this morning (that's right, I woke up early to sensitize!) and it was a cloudy brown sludge. This had been used previously, probably 2 months or more, to sensitize about 10 sheets of 5x7" and put back into the bottle w/o filtering.

My "mother" bottle of 5% stock solution is still crystal clear. So, I'm wondering if it is the organic material in the carbon/dye-transfer tissues that caused it to detiorate.

- Will filtering through a coffee filter be sufficient to prevent this from happening again?

- What is your method for determing exhaustion, replacement, reusing of potassium dichromate sensiziter?

2nd, I am doing something a little different than just typical carbon tissue sensitization, that is, I have gelatin on a clear support (melinex) and I will be exposing through the base so that this support will be the final support for a dye-transfer matrix. This requires that the backside be clean, ergo, I tried brushing on the sensitizer with a foam brush.

- I know this is common with spirit sensitiazation, but what about w/ PD?

- Is a 1 minute pre-bath still advisable with this method?

Thanks in advance for your hallowed wisdom, ye fine carbon printers.

 

[Vaughn]

#1. Dichromate contaminated with organic material (gelatin) will not last - I doubt filtering will help much. It would depend on how efficient the filtering is.

This is one of the reasons I spirit sensitize -- less dichromate to toss out. Close to 100% of my dichromate goes onto/into the tissue.
You might be able to sensitize a large number of sheets, then freeze the dried sensitized tissue for future use -- thus getting the most out the dirchromate bath.

#2 No reason PD will not work with spirit sensitizing, as long as the spirit and PD are compatible.

I was going to suggest that if you used the tray method that you just wipe off the base of the dry sentitized tissue with a damp rag -- but I suppose the chance of scratching the melinex would be a problem.

 

[holmburgers]

Thanks Vaughn,

I'm not technically "spirit" sensitizing in that I'm not using a solvent, just water. But I'm tempted by the idea, and would prefer to keep using PD and not AD since it's also useful for reversal processing. Anyone explicitly using spirit sensitizer with PD, and if so, what solvent?

Has uneven sensitization ever been a problem with brushing on?

Lastly, you're probably right about just wiping the back of the melinex... I think I'm going to have to do that anyways, as some of the solution made it's way around to the back.

Learning... learning...

 

[holmburgers]

http://bostick-sullivan.invisionzon...opropyl-alcohol/page__view__findpost__p__8670

Here is a great post describing spirit sensitization w/ either dichromate. I'm definitely going to give this a go... it seems better in so many ways.

However, I do have one concern. With these dye-transfer matrices, it is necessary that the interior gelatin be sufficiently sensitive, and not just the top layer (which is all that's necessary in carbon-transfer). Is there any reason to believe that spirit-sensitization is particularly "surface acting" or will have trouble reaching the bottom-most layer?

 

[Vaughn]

I do not think there is any reason to assume that spirit senstiizing does not penatrate any less deep than tray sensitizing -- nor to assume that carbon printers only need to sensitize only the surface of the tissue...my prints with a thick relief shows that I sensitize and expose deep into the tissue. And my tissue is probably much thicker than yours (1.2ml of glop per square inch)

Carbon printers must make sure not to expose all the way down to the base, or else the image will not transfer to the final support (it will stick to the tissue support instead). But the tissue is sensitized all the way to the base (otherwise we would not have to worry about exposing to the base).

The question becomes (with any sensitizing method)...Does the dirchromate disperse itself evenly throughout the depth of the tissue -- and does any particular method do a better job of it? Soaking may not do any better job of it than spirit sensitizing...especially with thin tissue.

 

[holmburgers]

That is certainly encouraging, thanks again.

 

[gmikol]

I use potassium dichromate and acetone for spirit sensitizing. I mix up the required amount and then split it in half. I brush half the volume until it is completely absorbed, then repeat with the other half of the volume. I haven't had any problems with uneven-ness, but I haven't been brave enough to tackle really smooth uniform skies, either.

Be aware that potassium dichromate can only form about a 11-12% solution at room temperature, and any "sprit" you add will knock the dichromate out of solution. So if you need *really* strong solutions, you're better off with ammonium dichromate.

Also, keep in mind that acetone and isopropyl alcohol (I use 99.9% "electronics grade") are compatible with either dichromate, but ethyl ("denatured") and methyl alcohol is not. Both will reduce to form aldehydes, which will spontaneously harden the gelatin.

I'm inclined to agree with Vaughn...I'm not sure it matters if the sensitizer is uniform throughout the thickness of the gleatin. A fixed amount of dichromate will harden a fixed amount of gelatin. The rest of the unhardened gelatin washes out during development, no? Regardless of whether you're carbon printing or making matrices for dye transfer.

Another thing you may consider is a sensitizer-incorporated emulsion. Neither AD or PD are particularly good for this, due to a fairly rapid dark reaction, but a double-chromate salt has been suggested for this use, most commonly ammonium potassium chromate, formed by adding ammonia to a saturated solution of PD.

http://www.andreazalme.com/Pages/LabNotes/LabNote_005/LabNote_005.htm

But I found one reference on the B&S board which referred to the use of sodium carbonate to form sodium potassium chromate. It's certainly avoids the issue of having to find strong ammonia, if you don't already have it. The other option is an incorporated diazo sensitzer, as has been touched on in the UltraStable thread.

--Greg

 

[Hexavalent]

Hi all,
I am doing something a little different than just typical carbon tissue sensitization, that is, I have gelatin on a clear support (melinex) and I will be exposing through the base so that this support will be the final support for a dye-transfer matrix. This requires that the backside be clean, ergo, I tried brushing on the sensitizer with a foam brush.

Chris, you can add dichro directly to the wet glop mix (see Nadeau's book) - zero waste and complete dispersion. "Pre-sensitized" tissue does not have good keeping properties though - best dried with cool air (AC works), exposed and processed soon after.

2 cents Canadian

 

[Vaughn]

Once poured, my tissue takes 36 to 48 hours to dry (thick tissues!) -- not very practical for pre-sensitizing.

I'd have to run some sort of dehumidifier in a cabinet.

 

[holmburgers]

Thanks ye all,

I'm trying to avoid the incorporated sensitizer... it kind of creeps me out actually, since I usually store my glop in the fridge. Plus, I like to separate my toxic and non-toxic activities (...who doesn't, right?)

The talk of different sensitizers is very interesting indeed. Honestly, I didn't know those options existed (except diazo).

I can't say for certain yet, but last night I successfully exposed/etched some matrices and it appeared that my brush sensitization worked ok. So until something rears its ugly head, I think I'll keep trying this with the possible addition of some solvent.

Lastly, I was always under the impression that PD should never go above 5%.