View Full Version : Alum: Speed increase or split-tone?

12-20-2009, 09:13 AM
I have been experimenting with Alum (potassium and ammonium rather than chrome) for hardening the gelatine size on my hand-made paper. Formaldehyde is not so easy to get hold of in the UK, and Alum is far safer!

Soaking the paper in Alum certainly does harden the gelatine, it also acts as a good preservative for the paper. However the Alum has some interesting interactions with the cyanotype process.

In my first batches of paper, I added alum to the pulp and also to the molten gelatine that I used to size the paper. I noticed that if I exposed the sensitised paper for the usual time I use on shop-bought paper, the image was very over exposed. Unfortunately it is also really hard to wash out all of the yellow sensitiser stain.

I had also been using a wash of sodium carbonate (washing soda) to provide a blue-yellow split-tone to the image. With alum being present, I found that the image just bleached away rather than obtaining a split (frustrating!)

In my latest batches of paper, I have processed without alum and sized with plain gelatine.

The image below is a 'reference' exposure that provides a nice print on the alum-free paper and the sensitiser washes away easily. Once dry, I soaked the image in a solution of 5grams of alum in 1Litre of water for 5 minutes to harden the gelatine and act as a preservative to the paper; there was no noticeable change in the image once it dried again.

For a second experiment, I soaked a sheet of paper in the alum, let it dry, then sensitised it and exposed it.

For a 3rd test, I over exposed a sensitised alum-free sheet by one stop, then bleached it in a sodium carbonate bath to create a split tone. The paper was dried and then washed in an alum bath to preserve the image; again no colour changes were observed.

Attached files http://f295.f295.org/uploads/owl_ref_423.jpg (http://f295.f295.org/uploads/owl_ref_423.jpg)

12-20-2009, 09:14 AM
This is the image from the paper that was pre-soaked in alum; the exposure was the same as the reference image, however the overall speed seems to increase, but the contrast reduce. The yellow stain does not wash out so easily when the alum is present. The image also seems to be more 'cyan' than blue and the colour is much more greeny-blue than without the alum (the more alum you use, the more 'cyan' the image).

Attached files http://f295.f295.org/uploads/owl_alum_7596.jpg (http://f295.f295.org/uploads/owl_alum_7596.jpg)

12-20-2009, 09:19 AM
Here is the image that was exposed 1 stop more than the reference, then bleached back in a strong bath of washing soda (a good heaped teaspoon to a litre of water). The cyanotype has split nicely to give blue/ yellow and white in the image. The yellow sensitiser washes away easily and the final alum soak did not seem to alter the colour.

My feeling is that the alum pre-washed image did not split-tone as the apparent speed increase is not a real increase in the 'density' of the Prussian blue; thus a one stop exposure over the correctly exposed alum-based image is still not a long exposure and thus the really dense parts of the image that remain after bleaching were not present.

Overall I think I am going to stick to using the alum as the final wash to harden the paper as I can live with high contrast (controlling it with photoshop curves for the digital negs.), but not having the yellow wash out easily is a pain!

Best regards,

Evan Attached files http://f295.f295.org/uploads/owl_1_stop_split_1685.jpg (http://f295.f295.org/uploads/owl_1_stop_split_1685.jpg)

12-20-2009, 03:31 PM
Evan - why do you need to harden the gelatin at all? If the processing of the image and the subsequent life of the image does not anticipate a temperature above the melting point of the gelatin, then is the hardening necessary? I have been experimenting with different sizes for salted paper prints in the past week, and the gelatin size works well to sharpen the tones of the image by keeping the sensitizer from soaking in to the paper fibers too deeply. I did not use any hardener at all, and all of the literature for salted paper that I can find does not mention hardening the size.

Paper sized with gelatin for carbon printing must be hardened, as the carbon tissue must be soaked in water above the melting temperature of the gelatin in order to develop the image. I guess the question is then - does the alum treatment add or detract from the image substantially? If neither the look nor the longevity is affected in an important manner, then why use it?

I am intrigued by using sized paper for cyanotype. After my salted paper sizing, I want to see what the same size does for cyanotype. I tested 250 bloom photographic gelatin at a very low concentration (0.8%) and arrowroot size at 3% (weight to volume). In both cases, I got better results with citric acid added to the sensitizer in both cases - I suspect that the same would hold true for cyanotype. It seems to prefer an acidic environment. Do you notice a difference in apparent image sharpness between unsized (or lightly sized) paper vs heavily sized?

12-20-2009, 06:54 PM

I must confess that the thought about not bothering to harden the gelatine has also crossed my mind. Alum also kills lots of bugs that could cause issues if the paper got damp which is actually my prime reason for the final treatment. Leaving out the final wash step does save time and also reduces the chances of tearing wet paper....

I give the paper an internal size of PVA glue when I make it; the last batch of paper I tried less PVA but the wet strength is compromised so I will go back to a larger quantity. I then apply 3 coats of about 3% gelatine (food grade). The water-leaf paper after it has first dried and has no gelatine is rather absorbent; it makes blotting paper look like a plastic sheet! I can get a good cyanotype after 2 coats of gelatine, but it needs at least 4ml of sensitiser to cover an 8"x10" sheet as it soaks in quite well. With 3 layers of gelatine, I only need 2ml of sensitiser to cover the same sheet. The problem is not the cost of the sensitiser, rather the paper is not always a consistent thickness and the exposure time needs to be a bit longer in the areas that get heavily soaked, leading to a blotchy image. The paper I made for the prints has a pH of 6.5 so is slightly acidic anyway; practically I found nearly all the differences in printing to be attributable to the alum; I have no idea what is in commercial sizes so the extra acid added to the sensitiser (in cyanotype anyway) may be influencing something else. I plan to try papers ranging in acidity to see what happens at the extremes. From pH of 6 to 9 (when plain gelatine sized only) there seems to be very little difference. I have sulphuric acid to get a really low pH but I doubt the paper will be archival!

I have experimented a little with arrowroot and flour starch sizes but in both cases the paper curled rather badly; the flour surface was a dream to coat though so I may try again.

The biggest problem I have with sharpness is that the paper is very rough. Where the paper touches the negative, I get far too much detail when using laser-printed negatives as I can see the 600DPI dot pattern in the cyanotype! In the dips of the paper, the negative is not in contact and so I get a softer edge. For pinhole images the results are fine but I do notice the difference when printing a lens-acquired image.

Best regards,


12-26-2009, 08:36 PM

Your handmade paper is lovely and this is a beautiful image to print on it. All 3 of your examples are outstanding! I'm impressed your paper has the structural integrity to withstand soaking and handling as well as it does. The PVA does a good job!

I would be concerned that unhardened gelatin would dissolve (not melt) even in an ambient temperature water bath, although Earl suggests this would only happen in warm water. Gelatin sizing is usually hardened for gum printing, even though the print is developed in cool water; presumably this is the reason. Perhaps a few tests are in order?! :)

On a related note, there was a lot of discussion on the alt process photo list a couple months ago about using diluted PVA *sizing* made by Gamblin instead of gelatin to size paper for gum printing. I have tried it and it works well for me, most of the time. It is an easy, quick substitute for hardened gelatin. Just brush and go.

There is also a product sold by Twinrocker - an internal sizing (alkylketene dimer) that people on that list were thinking might make a good brush-on sizing for gum printing. Is this an ingredient you are familiar with? I've been thinking of ordering some....My handmade paper was always like blotter paper so my hat's off to you for overcoming that stumbling block.

happy new year!

12-30-2009, 04:41 PM

Thanks for your nice comments about the prints.

You are right that the gelatine will dissolve in warm water. I develop in water straight from that tap that is about 15 deg.C and the print is pretty robust, although you can tell that the surface is starting to go a bit slippy by the end of the development. I did a brief test of gum on this paper, but it was not that successful though as the particular sheet was on the thin side; interestingly it was the highlights in the centre of the image that fell apart as the gelatine in the shadows around the edge seemed to have been hardened nicely by the dichromate leaching out of the gum.

I have not had good success in the past with diluted PVA on its own as a size for cyanotype, however I am planning a test with PVA then gelatine for the final layer; it should allow a warmer temperature to be used. The tests I have done in warm water where the paper survived still gave good prints, even though you could see the gelatine washing away! The cyanotype mix does penetrate into the paper a fair way.

With the gelatine size, I get the deepest blues I have ever had an no issues at all with blue washing out and staining the highlights. For exposure on the commercial paper, I waited until the shadow areas solarised and went brown and the highlights were no longer yellow but looking distinctly green. In the wash much of the blue 'floated away' but left a good image. With my paper and a good exposure, the image is only just visible and the shadows are 'green' with the highlights still a bright yellow; any areas that get enough UV to solarize to brown develop so dark it is easy to mistake it for black.

Best regards and Happy New Year,