A bit of history: I started out sizing, because Keepers of Light recommended it, but one day a gallery called and said they'd sold four tricolor prints and wanted four more right away to replace them. I happened to be out of sized paper at the moment, so on an impulse I grabbed some paper right out of the box and started printing. I made four tricolor prints (including the print of apricots on my home page) on the unsized paper, with no staining whatever, and from then on until the end of the century, I printed everything on unsized Arches. But then Arches changed and suddenly I was getting speckling in the prints and weird anomalies in the paper, and because of these changes and inconsistencies, I stopped using this paper altogether. In the interim I experimented with different papers (unsized Lana aquarelle, unsized Fabriano Uno (no longer available) and some other papers, and finally settled on Arches bright white (a different paper from the traditional Arches watercolor paper), which like the current version of Arches aquarelle, doesn't print well unsized. I size it with gelatin hardened with glyoxal.
Hence my qualified answer: "it depends." If your paper will print well unsized for you, then don't bother. If it won't, then size. It's as simple as that.
Types of sizing I've used:
Gelatin and gesso: My favorite size is one I hardly ever use: gelatin mixed with gesso. It gives a lovely smooth surface for printing on and seems to print especially cleanly and brilliantly (both very subjective judgments). The reasons I don't use it routinely are: (1) I don't know anything about the archivality of this combination, so I'm a little leery of using it for exhibition prints, and (2) it's messier and requires more cleanup than usual sizing methods. But if you're interested in trying it, I used a package of Knox gelatin in about 125 ml water, added a couple of good-sized glops of gesso and mixed well.
Acrylic medium: some people do very well coating a sheet of paper with acrylic medium as a size, diluted anywhere from 1:4 to 1:10. I've had mixed results with this, so I haven't used it routinely.
Unhardened gelatin: There has been some unwarranted ridicule in some quarters about using unhardened gelatin as a size, but there's nothing in the method itself to warrant the ridicule (hence "unwarranted ridicule"). If there is some exposure throughout the print, the underlying gelatin size will be hardened by the dichromate in the presence of UV, at the same time the gum is hardened. If there are unexposed areas in the print, such as large white areas, then there might be some concern about the unhardened gelatin in those areas eventually attracting bugs; if that's a concern, the unhardened gelatin in those areas can be easily removed by rinsing the finished and dried print with hot water. For more information on these observations, with demonstrations, see the separate page on sizing with unhardened gelatin.
Most sizing recommendations for gum consist of gelatin hardened with some sort of hardener, to reduce mold and the swelling and slipperiness of the gelatin that occurs when unhardened gelatin gets wet. There's a lot of preciousness in some quarters over which hardener is "best," but looking at the work of people using different gelatin hardeners, I don't see a discernable difference in smoothness or in lack of speckles or blotchiness between the different hardeners.
Glutaraldehyde: Some people really love this hardener, but I've had nothing but bad experiences with it. It is a fairly new hardener and hasn't been widely used, but already on the alt-photo list at least 8 or 10 people have reported unpleasant reactions to the fumes. I had such a bad experience breathing the fumes that I required medical attention; after that experience I would be very leery of recommending this material to any gum printer.
But it's not just the possible toxic effects of the material; I didn't like it very much as a size either. It printed very grainy and flakey for me, compared to the very same emulsion on unsized paper. The image would develop normally for a while and then suddenly would disintegrate and flake all into pieces:
Since I had such a bad reaction to the material, I didn't size very many pieces of paper with it so haven't tested it extensively, but all things considered, I'm not interested in pursuing it further.
Glyoxal: This is the hardener I've settled on for my work on Arches bright white, and is probably the hardener that's used by the majority of gum printers. I've never experienced any unpleasant reaction to it; it's easy to use, and gives a nice print.
A big deal has been made in some quarters of the fact that glyoxal-sized paper can turn yellow after a few days, but Judy Seigel demonstrated that if you rinse the paper after sizing, that yellowing doesn't occur. I've found that I don't need to rinse at the time because even though the paper does turn yellow, the yellow completely rinses out of the paper during the development phase of normal gum printing. I don't know if this is true for all papers, but it is true for the Arches bright white in my practice.
I just offer this one as an idea; I once tried using gum hardened with glyoxal as a size and got interesting but inconsistent results.
Sizing the paper, how I do it:
I sprinkle 1 package of Knox gelatin into 200 ml water, allowing it to swell for 15 minutes or so before stirring and heating. I heat the gelatin in a microwave to about 120 or 130 F, stir well, and add 3 ml glyoxal (the stock 40% glyoxal you get from suppliers such as Photographers' Formulary).
I set the gelatin container (a glass measuring cup) in a tub of hot water to keep it warm while brushing on the paper. I have found that it's important to keep the temperature below 140F, because for paper like the Arches bright white, which is factory-sized with unhardened gelatin, if you use a liquid on the paper that's too hot, you can melt the internal gelatin size and wreak havoc with it, creating little holes that cause speckling in the gum print later. This isn't true for all papers, I think, but it's certainly true for the Arches papers in my experience. So I check the temperature every now and then and keep it between 125 and 135. I brush the gelatin on just the front side of the paper, letting the gelatin soak in and then brushing on another layer, to make sure the gelatin is well and evenly soaked into the paper, Then hang to dry.
Copyright Katharine Thayer, all rights reserved.