sodium dichromate for gum printing?

[phritz phantom]

i finally have the time again to start gum printing, after a long time.
unfortunately the sensitizer of the fotospeed kit i once bought is running out and getting dichromate (without a license) is getting pretty hard in austria/germany. i hope i can order 100gr of ammonium dichr. one more time.
all the german foto shops seem to stop carrying raw chemistry and the chemical suppliers won't sell it to me (neither local ones).

so another possiblity that i can keep up doing gum prints would be using sodium dichromate, which is sold by wolfgang moersch.
can this be used for the gum process? in which way does it differ from ammonium/potassium dichr.? also in levely of toxicity, danger...?

 

[sanking]

Yes, you can use sodium dichromate for gum printing. It can be mixed in higher percent solutions than ammonium and potassium dichromate, but its deliquescence can be a problem in measuring.

Sandy King

 

[phritz phantom]

Yes, you can use sodium dichromate for gum printing. It can be mixed in higher percent solutions than ammonium and potassium dichromate, but its deliquescence can be a problem in measuring.

Sandy King

do you mean measuring for making the solution?
that wouldn't be a problem, because moersch sells it in form of a 50% solution.
not having to handle the powder is another point for it.

 

[sanking]

do you mean measuring for making the solution?
that wouldn't be a problem, because moersch sells it in form of a 50% solution.
not having to handle the powder is another point for it.

Yes, I did mean measuring for the solution. It should be fine if you buy it already in solution.

Sandy

 

[phritz phantom]

then my gum printing is hopefully saved for the next years.
thanks a lot, sandy.

 

[phritz phantom]

i have one more question. i was able to order some ammonium dichromate, so i'm saving the sodium dichromate route for next time.
i don't have my chemical scale at this place, so i'm stuck with making a saturated solution. if i had a scale here, i'd just put 30gr in 100ml of water and get a 27% saturated solution with some leeway.
making a saturated solution is considered a way around using a scale? how does one do that? under which circumstances should the dichromate stop dissolving? i mean circumstances like: temperature of the water, agitation, time ...

maybe i'm taking things too complicated right now, but somehow my head won't wrap around it. please tell me how you do it.

edit: also, could i also use acetone instead of water to make the solution? to improve drying time.

 

[Lukas Werth]

Saturated solution:
am.di. should be around 30%. Just measure about 35 g to 100 cc, mix, warm (dichromates react endothermic when brought into solution), remix, do so for a while, and there should be still something left which refuses to go into solution. Then the solution is saturated.
Sod. di. is much cheaper than am.di. (as is potassium di.), and works as well. You have to look around for chemical retailers willing to sell to private customers, or try shops selling teaching equipment ("Lehrmittel") - or order it internationally, from Aldrich or wherever. Anything is likely to be cheaper than your source.
While Sandy is theoretically right with his caution, I don't think it matters practically. Differences in contrast and exposure time occur between, say, 10% and 25%. Or bring everything into solution at once.

 

[phritz phantom]

thanks, so i'll need a scale nevertheless. i do have one, but it's 300 miles away right now, so i hoped to somehow being able to avoid buying a new one.

i used to get dichromate from brenner, but they unfortuately seem to have closed down their (raw) chemistry section. i got some from foto mayr now, but i doubt they'll keep selling it for a long time. no luck with chemical retailers, yet.
moersch sells sodium dichromate as an additive for oxalate-processes, so this might be the most reliable (for the future) source of dichromate in germany for me.
http://www.moersch-photochemie.de/chemie.php?typ=sonstiges (bottom of page)

one thing more, that interests me:
in carbon printing sensitizing is done after the pigment is applied to the paper. would this be possible with gum printing too?
first coat a layer of pigment and gum, then (shortly before printing) coat another layer of pure dichromate. i can't see why this wouldn't work. most chemicals of the two processes are the same, the only difference is the gum.

 

[donbga]

thanks, so i'll need a scale nevertheless. i do have one, but it's 300 miles away right now, so i hoped to somehow being able to avoid buying a new one.

i used to get dichromate from brenner, but they unfortuately seem to have closed down their (raw) chemistry section. i got some from foto mayr now, but i doubt they'll keep selling it for a long time. no luck with chemical retailers, yet.
moersch sells sodium dichromate as an additive for oxalate-processes, so this might be the most reliable (for the future) source of dichromate in germany for me.
http://www.moersch-photochemie.de/chemie.php?typ=sonstiges (bottom of page)

one thing more, that interests me:
in carbon printing sensitizing is done after the pigment is applied to the paper. would this be possible with gum printing too?
first coat a layer of pigment and gum, then (shortly before printing) coat another layer of pure dichromate. i can't see why this wouldn't work. most chemicals of the two processes are the same, the only difference is the gum.

You don't really need a scale to make a saturated solution of am. di or pot. di.

Simply follow Lukas's directions sans the measured dichromate.

IOW, keep adding dichromate until you have dichromate crystals deposited on the bottom of your container. I maintain a saturated solution by adding water and/or dichromate as I use it up.

 

[Katharine Thayer]

one thing more, that interests me:
in carbon printing sensitizing is done after the pigment is applied to the paper. would this be possible with gum printing too?
first coat a layer of pigment and gum, then (shortly before printing) coat another layer of pure dichromate. i can't see why this wouldn't work. most chemicals of the two processes are the same, the only difference is the gum.

But be that as it may, it doesn't work, and I'd caution against generalizing too much from one process to the other, since they are different in many important respects.

The reason this doesn't work for gum though it works for carbon is that unlike gelatin, gum is hypersoluble in water at room temperature. I think on my page about the mechanism of the process, I show the effect of simply dropping a drop of water on a layer of dried gum/pigment; it dissolves the gum/pigment instantly when it touches it. And I expect most gum printers have had the unhappy experience of ruining a coated paper (pre-exposure) by inadvertently getting a drop of water on it; I certainly have.

Somewhere on my site I'm sure I have a demonstration of this method -- I mean of coating first with gum/pigment, drying, and then sensitizing with dichromate. I've just searched the site and can't find it, but trust me, it's not a pretty sight. The liquid sensitizer dissolves the gum/pigment underneath and smears it around. The print is recognizable, but the coating is all uneven and streaky because of the redissolution of the gum; unless you want to try it for some special effect value, I wouldn't recommend it.
Katharine