March 2009

Monthly Archive

Busy With Teaching

Posted by on 29 Mar 2009 | Tagged as: Gum Bichromate

I’ll be teaching full-time for a few weeks now, and don’t have much time for alt-photo work of my own. However, I’ve been thinking of gum printing too, and here are a couple of things that I must consider to get students involved more easily:

Paper: Got to find good, dimensionally stable paper with good wet strength.
Sizing: Thought I could manage without extra sizing, but it looks like it’s just necessary to size certain papers. Also, I need to determine which sizing method to use.
Pigments: Finding an assortment of good quality non-staining pigments.
Negatives: Proper correction curves; it may be necessary to generate separate curves for each color (in 3 and 4-color work) instead of single generic curve common to all colors.

By the way – good news about Varn gum! We can get it again – our supplier was able to make a special order from Germany. It will make our life a little bit easier, with no need to mix the gum from powder or lumps…

Gum Inconvenience

Posted by on 18 Mar 2009 | Tagged as: Gum Bichromate

In gum bichromate printing we have been using Varn 14 Be lithographers gum solution, bought in 10 liter containers. From our supplier we heard that it is not available any more, and they sent what is supposed to be a substitutive product, “Agum O” solution by Hanns Eggen GmbH, Germany. The supplier assured us that it would be the same stuff, with same properties as Varn had.

But it isn’t! I made some tests with my standard pigment/gum/dichromate mix that I had used with Varn gum.

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Some step table tests with “Agum O” gum and Schmincke and Linel pigments.

In my quick tests with step wedges I got very stained results. No matter how I mixed the sensitizer my pigments always stained the paper base heavily.

I tried to google for some information on “Agum O” but couldn’t find much. This gum feels a bit watery compared to Varn. Some sources refer to it as synthetic gum, and on this Finnish web page it is said to contain also dextrine in addition to gum arabic.

I couldn’t use this gum! So I decided to mix my own. I bought gum arabic as fine powder and mixed it with water at 1 + 2 (100g gum + 200ml water). The solution became somewhat thicker than the standard 14 Be gum, but some gum printers prefer it at this strenth.

I will need to recalibrate my gum printing workflow, so I guess it’ll take a while until I get some results posted here…

Down to Gum Business Again

Posted by on 06 Mar 2009 | Tagged as: Gum Bichromate

After a few weeks winter break it’s time to get back to gum printing!

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This picture was one of my test images when trying the curves introduced in the previous gum printing post. The picture was taken with a Nikon D70, it is a RAW file, and has been slightly processed by increasing color saturation and vignetting the corners a bit. Cyanotype was used as the cyan layer.

Color separation was made by splitting RGB channels into three separate grayscale images. A correction curve created by ChartThrob was applied to each image. The images were then inverted to negatives and printed on CopyJet transparency. The negative from R channel was printed in cyanotype, B in gum bichromate with yellow pigment, and G with magenta pigment, in this order.

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The three separation negatives.

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The first two print layers: Cyanotype and Cadmium Yellow.

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Jalo Porkkala: Samos Wall, tri-color gum / cyanotype.

Magenta as the topmost layer.

The outcome is a bit powerless and muddy – color balance and saturation are not quite there yet – the Maimeri pigments chosen may not be the most adequate ones for the job. Also printing each color layer twice could yield better color depth.