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Writer / Geoff Chaplin
Photography / Geoff Chaplin

Testing paper sizing for gum prints

Is sizing needed as a complement to your working method? Follow the tests to determine if it is needed.

Watercolor paper contains a 'size' to limit the penetration of pigment into the body of the paper. Some other types of paper - such as Arches 88 - contain no or very little sizing and act like blotting paper if put in water. Such papers must be sized before use. Whether the amount of size in watercolor paper is sufficient for your particular application and methods is something only you can determine - key variables being the number of layers you print, the pigment densities and the development method (still, light or heavy brushing). The consequence of too little sizing is significant amounts of pigment staining and peppering in areas which should be clear. Sometimes this is a desired effect - the following two illustrations where there are large areas of plain white background show heavy (deliberate) staining on an unsized paper compared with a relatively clear background on a sized paper.

Image 1: Three layer print on unsized Saunders Waterford NOT 300g		Image 2: Two layer print on sized Cotman Take Bristol HP 230g

Is sizing necessary?

Tests in this section show portions of "clear" areas of prints made using the same negative as Image 2 above. In performing these tests paper was coated in batches using the same pigment/gum/dichromate mix (10g gum, 1g pigment, 10g 5% dichromate), with the same brush and as similar a coating method as possible. Paper was dried for 30-45min at 20 °C and printed for the same time over a UV florescent light source, then developed in water at 20 °C for 30 minutes. Fabriano Artistico HP 200g paper was used and three pigment layers - yellow ochre, Van Dyke brown, and 60% burnt umber with 40% lamp black - were used.

The first set of tests used still development and shows - to my surprise - significant and similar staining on all three sets of paper.

Test 1: unsized, still development (detail from a largely blank area)		Test 2: single sized, still development		Test 3: double sized, still development

In prior staining tests I found that 10% levels of these pigments cleared from sized paper (leaving a marginal amount of stain) after 30 minutes still development, although the tests were performed on a different paper. The above results show significant staining in all three cases.

I then repeated the first two tests but using light brush development, which is my normal method - still development for 25 minutes then gentle brushing under water.

Test 4: unized, brush development		Test 5: single sized, brush development

Here we can see a significant difference in the results with the sized paper clearing well but the unsized paper leaving a general stain and peppering of pigment.

Test 6: unsized		Test 7: sized

One other result to emerge from the tests is that the sized paper requires more exposure than the unsized paper for the same image density. The following two crops are taken from a different image but where the exposure and development was the same - the sized paper showing a much thinner image.

Sizing has different impact on different papers not only being related to the internal size in the paper but also the texture of the surface. I find very smooth surfaces - such as Fabriano Artistico - tend to hold the printed gum layer less effectively the more they are sized. This is particularly apparent when using heavy pigment concentrations.

In addition the relationship between sizing, the pigment and density used, and the brushing and development methods are complicated: the reader will find it instructive to do pigment clearing tests on both sized an unsized paper. To do this for a chosen pigment and density paint a small amount of the gum/pigment/dichromate mix onto the paper and allow to dry. Then (without exposure) follow your washing procedure - doing this for a variety of pigments and densities will tell you what clears with what paper and sizing.

Finally note there are other sizing methods: gum/dichromate can be used as a size as described in Christopher James, The Book of Alternative Photographic Processes. I find single sided sizing (whether by gum or gelatin) tends to increase paper curl so more work has to be done to keep the paper flat.

Sizing method

The following description is my implementation of the methods described in Scopick, The Gum Bichromate Book and Crawford, The Keepers of Light.

1Tear the large sheets of watercolor paper to appropriately sized sheets and mark the back in one corner in pencil - this tells you both which side to print and is also useful in the sizing operation below.

2Pre-shrink about 20 sheets of the paper for about an hour in initially hot (40 °C) water then hang to dry overnight.

3Size the paper using a 3% gelatin solution. Gelatin can be raspberry flavour from the grocers which tends to be fine grain and dissolve quickly and easily (and tastes better), or photographic grade which is coarser. Slowly add 45g to 750ml of cold water and allow to stand for 10 minutes or so. Then top up to 1500ml with hot (60 °C) water and stir thoroughly. Prepare two trays - one containing hot water (60 °C), the second empty but sitting in the first. Pour the gelatin into the upper tray and cover with a sheet of cardboard - this keeps the heat in. Occasionally roll the liquid up and down and across the tray to assist in dissolving the gelatin. After about 30 minutes it should all be dissolved. Do not use the size until all the gelatin is dissolved.

4Treat about four sheets of paper at a time - making sure the water in the lower tray stays hot enough for the gelatin solution to flow quickly and easily over the paper. Put each sheet in individually and make sure it is thoroughly covered before adding the next, and soak for a minute then remove and hang to dry vertically. If the solution remains warm (circa 40 °C) then excess solution readily flows off the surface and does not leave any bubbles.

5Repeat until all 20 sheets are done, then cover the tray with the cardboard (to keep dust out) and leave till tomorrow. The quantity of solution will be down to about one litre.

6Next day refill the lower tray with hot water to melt the solidified gelatin. Once melted refill the lower tray again (it will have cooled) with hot water and repeat the sizing of the 20 sheets after adding 1.5% glyoxal (as a hardener) to the remaining solution. Hang the sheets to dry vertically but in the opposite direction to that done the day before. This supposedly equalizes the thickness of size and means the paper is uniformly tolerant of a certain pigment density before staining though I haven't actually tested the truth of this. I suspect if the size is warm then a single coat probably leaves a uniform (albeit thinner) layer because it flows off quickly. The gelatin solution will be down to about 500ml - which can now be discarded.

The article is written in the context of gum printing though the sizing process is the same for all print methods.

See also Geoff's gallery for more of his work.

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