View Full Version : Chrome alum

02-03-2011, 01:29 PM
I need this alum to harden the emulsion (Fotokemika VC, same as Rollei BM3) applied on stretched canvas, but can't find it anywhere, except imported here to Croatia at outrageous prices. Several suppliers even told me it is classified as poison and won't import it. Would anyone be so kind to part with a tiny portion of their stash?

02-03-2011, 01:48 PM
PM sent.

02-04-2011, 06:51 AM
First of all, thanks to Paul, who kindly offered to send me some.
While on the subject, can $140 for 250g of pure chrome alum be considered a steep price?
Also, should it be added to the emulsion while liquifying/diluting or only to the gelatin coating?
I'm using good quality artist canvas, primed with non-oil primer.

02-04-2011, 07:58 AM
Something's weird there. I bought chrome alum without any problems from my usual chemistry supplier. It wasn't especially expensive. It's not toxic, or regulated in any way AFAIK. It's not labelled even irritating here, but I can see that some MSDS's state that.

140 USD for 250g sounds quite high. I paid 39,80 EUR for 500 g and I've understood that generally you have lower prices there. I would have bought less but 500g was the smallest bottle.

It can, like other hardeners, be added to the melted emulsion itself just before coating. Just make sure to mix it thoroughly. I've not tried it yet, I've been using glyoxal as a hardener.

02-04-2011, 01:55 PM
In the US, in the Chinese & Asian food markets (probably elsehwere too), you can by Alum which is used in something related to food. In other words, it's cheap and relatively safe. It is also used in styptic pencils which stop bleeding.

Chrome alum, now I don't know if it's identical to alum, but only costs $37.95 per pound in the US. In other words, the price you were quoted is ridiculous!

I'm not sure if grocery store alum can be used as a hardener, but there might be some discussion about this on APUG and it's worth exploring.

edit: Ahh, here you go, "If it's for food use, it's most likely potassium aluminum sulphate - not 'chrome' alum.
K Alum will harden gelatin, but not to the extent that chrome does. Both perform better with the addition of some acid." from http://www.apug.org/forums/forum42/81120-paper-gelatin-alum-sizing-multitude-recipes.html

02-04-2011, 03:19 PM

You better check every time you talk about chemicals. Having common parts in names does not mean anything. Furthermore, the "old" trivial names are very misleading. The "Alum" part in "Potassium Alum" comes from aluminium, so the name is somewhat rational; but "Chrome Alum" is a similar compound with chromium instead of aluminium, so it should be named "Potassium Chroum" or something like that with the same logic :D. But instead, it has the "alum" even when it does not contain aluminium!

Long story short, every time check what's the real chemical behind the name. There may be a difference in just one letter (a common example is sulfite vs. sulfate). This time we are lucky because although the names are misleading, they are not unambiguous, which is the case sometimes. They just need to be considered as "tags" instead of descriptions of these chemicals.

This is a bit sad part of chemistry, but no can do; it's the burden of the history. In the same way, the "unimportant" cation has given name to many compounds such as soda, potash etc.

02-04-2011, 03:26 PM
Agreed, and lesson learned. But I never claimed they were the same and indeed said that. If alum (potassium chromum...) has hardening attributes then I feel that my post was justified.

02-04-2011, 03:49 PM
"Alums" are a class of related compounds - usually double sulfates of metals (aluminum, chrome, iron, maybe zirconium?) that form similar crystals. The "common names" are often misleading. Potassium Aluminum Sulphate (avail in the spices section of grocery stores) is the same as what is found in the 'hardener' concentrate of some fixers - it will harden emulsion in an acidic environment.

02-04-2011, 04:21 PM
I'm pretty sure P.E. had something to say on the use of "food alum" - Something to the effect that whilst it *may* work, better results were to be had with chrome alum (CAS No. 7788-99-0).

Photo Engineer
02-04-2011, 04:40 PM
You should not use Alum (white powder sold in grocery stores and for film and paper hardeners) for hardening emulsions that are going to be coated. You can use it in food. Ian is correct in that there are several "alums" including Zirconium which is called Zircotan.

Alum, or Potassium Aluminum Sulfate is rather harmless and has a LOT of pucker power. If you ingest it, your esophagus may close off but you will not be poisoned. Go to a poison control center or hospital.

All of the other Alums are poisonous. If you ingest any of them, go to a funeral director as quickly as possible. (Just kidding, but it is not good. It is much worse than any of the others.) Chrome alum is bright blue, kinda like grape juice. It is the worst of a bad lot.

None should be banned AFAIK and that price is very high. Check the price at the Formulary.


Kirk Keyes
02-05-2011, 12:37 AM
"Potassium Chroum"

I like that. Hard to say, but still cool.

You should submit that to IUPAC.

02-05-2011, 12:53 AM

I've seen many dilutions for chrome alum suggested for hardening gelatin - Anything from 0.025% and upwards. What would you (PE) recommend for general usage ?

Photo Engineer
02-05-2011, 09:41 AM

Chrome alum can be used to harden both raw emulsions before coating and as a hardener during processing. This solution is quite toxic. Handle with care.

For coating, I make a 10% solution in water and add about 5 ml / 200 ml of 10% gelatin. The hardness is about the same over that range, but the length of time needed to achieve good hardness varies from 24 hours to about 96 hours.


02-05-2011, 11:20 AM
Additional question; I've got an impression that chrome alum works by a slightly different mechanism than formaldehyde or glyoxal.

So, can I get even more hardening if I use both chrome alum and glyoxal in the same emulsion, if they both work at the same time by their own mechanisms?

Not sure though if I need more hardening.

Photo Engineer
02-05-2011, 01:52 PM
Yes, the Chrome Alum and Alum hardeners act by forming an ionic bond with gelatin. The Aluminum based Alum can be reversed and the film or paper can become softer, but I am not sure about Chrome Alum due to its strength. In addition, Chrome Alum forms a bond with glass plates.

Formaldehyde forms a covalent bond with gelatin and once formed it is very difficult to reverse! So the bonds are totally different in nature and in effect. As you go from formalin to glyoxal to succinaldehyde to glutaraldehyde, you also form a "longer" bond length which changes the characteristic of the hardening.