Gum Coating Problems: "Fisheyes"

This is a blurry shot with my cheap digital point and shoot; maybe I'll replace it someday with a sharper image, although fisheyes aren't something that happen to me routinely and I had to go out of my way (by coating on Yupo and by adding water to the mix) to create these for this page. But when I posted this online, people recognized it as the kind of fisheyes they've been troubled with, so I'll use it unless/until I make a better one.

Symptoms: When coating, gum retracts laterally from areas of the substrate, leaving holes in the coating. These can be as small as pinpricks, or as large as 1/2" across, but whatever the size, they appear the same way, as if the coating is retreating from that area of the paper.

Cause: In my experience, fisheyes are almost always caused by a problem with the substrate or the sizing, either a too-hard or too-thick sizing, or inconsistencies in the sizing, or a too-hard or too-slick surface (as above). It all relates to the same principle: gum needs a certain amount of "tooth" to hang onto, and if there's no tooth (as in the Yupo) or if the gelatin fills up the tooth and leaves nothing for the gum to hang onto, even in small areas of the paper, the gum can't adhere there, and retracts laterally from the area, leaving holes in the coating.

Once in a while, the cause is that the coating mix is too watery and/or sloshed on too thick, (that's what I had to do to create the fisheyes above even on the plastic Yupo) and the excess water keeps the gum from adhering to the available tooth.

I have seen no evidence, in all the years I've printed gum with many different pigments, to support a recent assertion that magenta pigments create fisheyes more than yellow or blue pigments, or that there is any relationship between pigment and fisheyes. Also, I never saw fisheyes in all the years I printed on unsized paper; I've only seen them (very small like pin pricks) on sized paper.

Longterm solution (Prevention): Use less gelatin in the gelatin solution (some people recommend as little as 1%); use a softer gelatin (it's my suspicion that a harder gelatin can contribute to the problem); add defoamer to the gelatin (a suggestion from Judy Seigel); make sure your gelatin is warm enough to go on liquid-- if it's a gel when you're brushing it on it could cool too thick on the paper and fill up the available tooth.

The main thing to keep in mind is that gum adheres to the paper, not to the size; the size is just there to keep the pigment from sinking into the paper. So you need a sizing that soaks into the paper but leaves tooth at the surface for gum to hang onto, and isn't so slick or hard that it interferes with the adhesion of the gum to the paper.

In the case of a too-watery coating, of course the answer is to not add so much water in future.

Immediate solution: in many cases, especially if the problem is partly or wholly that the coating mix is too watery to begin with, or if the fisheyes are small, if you continue brushing the coating, you can brush out the fisheyes as the coating becomes drier and more viscous. In other cases, especially if the coating mix is not too watery and the coating is drying before the fisheyes can be brushed out, the best solution is to wash the coating off the paper, dry the paper, and try again later.

Copyright Katharine Thayer, all rights reserved

Process Notes