Problem: Coating is streaked or uneven.

This is a problem I almost never have, as I live and work in a humid climate (RH almost always > 90%) and so my coatings (1 unit gum/pigment: 1 unit saturated ammonium dichromate solution) brush on easily and smooth out evenly with a few whisks of the smoothing brush. But if you live in a drier climate, you may experience this problem of the gum setting up too fast to smooth out evenly. Here's an example I got of uneven coating on a day of unusually low humidity. (Ordinarily if I had a coating like this that couldn't be evened out, I would just wash it off, dry the paper and start over, but this time I went ahead and printed the uneven coating to show how the unevenness reflects in the print):

Symptoms: Brush drags when coating; coating difficult or impossible to brush on evenly or smooth out.

Cause: Low ambient humidity results in gum setting up too fast while coating.

Possible immediate solutions:

1. Wash off coating, dry paper and start over.

2. Dip coating brush in water (very quickly) and smooth out coating. This may or may not work; if you add too much water you may introduce other problems, and if the gum has set up too much before you add the water, the wet brush may simply pick up the gum layer in places rather than smoothing it out. So this remedy has a fairly low probability of success, but if it doesn't work, you can revert to (1).

Possible longer term remedies, or preventive measures:

1. Add liquid to your coating mix to make it easier to brush on. Some add water, some use high-proof alcohol. People who routinely add water to the mix may never have this problem, but I prefer to keep the pigment and the dichromate saturated (click link to see a test that explains why I prefer saturated dichromate to diluted dichromate). Be aware that if you add liquid, the emulsion will be slower and the pigment less saturated in color than if you printed without added liquid.

2. Some have found coating with a fine-grained foam roller to be the answer, keeping the gum undiluted and rolling it on rather than adding liquid to make it easier to brush on. I've never been able to make this work for me, but others have been very successful with it.

3. Dampen your paper prior to coating, blotting the paper to remove surface water before coating, and drying the gum layer before printing. I've tried this with good results, and this is the path I would pursue if I encountered this problem often, since I would rather not dilute either the pigment or the dichromate by adding liquid to the mix. Another possibility might be to humidify the paper rather than wetting it, which some gum printers (Keith Gerling) have found to work well.

Copyright Katharine Thayer, all rights reserved

Process Notes