Gum Bichromate Emulsion Coating Stories...
I'm fairly new to gum printing and therefore making all the mistakes possible all at once. Partly due to what may be the inherent unpredictability of gum printing, and partly because I'm impatient and constantly change multiple things at once.
So. For you new gummists and others who may have problems getting good and consistent coatings - I thought I'd relate my experiences and solution.
I'm based in sydney which (of late) has been relatively dry - so it has be hard to get a good even emulsion coating.
I started by slowly and gently foam brushing a coating which seemed like the best way forward, according to the interwebs - resulting in FAIL.
Heres how it went…
Method : Foam brush, gently and slowly in even strokes, first down then across.
Result : Gum set up too quickly and didn't smooth out, streaks, fisheyes, gum inconsistently washed off the paper depending on how thick it was in that area.
Method : Hake brush, gently and slowly in even strokes, first down then across.
Result : Same thing. Gum set up too quickly, streaks, fisheyes, gum inconsistently washed off the paper.
Method : Foam Roller, down then across in relatively quick movements.
Result : Great!… for the first roll. Then as the roller gets wetter, it gets more inconsistent and has a nasty texture. Emulsion sets at different thicknesses over the print area and washes off inconsistently. again.
Method : Screen printing screen and squeegee.
Result : God I thought I was clever doing this. It even looked great till I peeled the paper from the screen… The emulsion is too runny for this to work. Emulsion pools and streaks as you remove it from the screen. (I really wanted this to work... if anyone has tried it and got it to work - let me know)
Method : Glass Coating Rod.
Result : Once again. Feeling very clever. I got pretty good results really, but it was messy, its hard to get consistency, I kept dropping and dripping and so on. The paper buckles and the emulsion pools before it sets up. Don't bother.
Method : Get depressed and go back to reading.
Result : Read Walter Zimmermans gum printing guide from Photo Miniature 1910 they talk about but don't link to on Bostick and Sullivan tech pages. (Here it is on google books)
Get excited again and try something he suggests…
Method : Foam brush emulsion application and a dry hake brush for smoothing.
Result. AWESOME. Even, consistent, smooth, no fisheyes.
Get the emulsion on QUICKLY with the foam brush. You can be a little splodgy and imperfect at this stage. Then, as soon as you have finished, with even more urgency, just as Zimmerman describes, use the dry hake brush like you see archaeologists brush away dust from fossils. Its all in the wrist.
Whew. I was going mad.
Really. Its hard to learn all these techniques from written accounts only. I wish there were more videos on the web.
Anyway. I hope this helps someone. (I think its partly me just barking like a lonely dog into the night - I haven't got anyone to talk to about my gum obsession)
Thanks for reading
Thanks for the account . I live in an area where low humidity is never a problem. Just the opposite. I have to use a hair dryer on my coatings before printing! It will be good to have an idea of how to proceed should I ever coat in low humidity.
My tool of choice is a 6-inch varnish brush, well-loaded on the first pass, then picking up excess and smoothing out on the second -- each done as quickly as possible without losing a smooth, even motion. (Like you noted: there are manual techniques that are hard to describe in words.)
Welcome to APUG and best of luck and fun with your gum adventures.
Kudos to you Benjamin for sticking with it. I know when I ran into similar stumbling blocks when I 1st started gum printing, I gave up and took the next workshop I could find. While I'm not a gum-a-holic, I do love the process.
Things that effect my coating:
1. Pigment concentration - I find I can coat easier with around a maximum of .8gr pigment to 10ml gum.
2. Pigment color - Ivory black and neutral tint seem to make a very thick emulsion that can get sticky before I can even finish the brush stroke. So I use less pigment and maybe even add a bit of water.
3. Paper & Sizing - My favorite paper Rives BFK, can suck the moisture right out like a sponge. A good layer of gelatin sizing makes things much more manageable.
I use a 3" hake brush for coating. Usually I don't smooth with a clean hake, but every now and then it seems the only way to get a good even coat.
Have fun and thanks for the story.
Congrats for the perseverence Ben. I would like to add (before you hit that wall): Later, you may need to add a little water (extending your usual mix with water, that is...), especially for large pieces. Large areas are prone to become tacky and to get unmanageable (before full coverage) very easily! As long as you keep the unextended emulsion amount per area constant, the extra water won't do much - because it will eventually evaporate before exposure...
Thanks for the tips - I'm just starting out with Gum Bichromate... trying to learn from books, which I'm not finding easy at all. Some of what you've said still means nothing to me, so I'm heading back to do some more research.
Well done for sticking at it Benjamin!
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I thought you all might be interested in my progress since my last entry. I also thought I might as well talk about my whole process as there are bound to be some people out there just looking for any information they can find.
I am working towards an exhibition in February of landscapes in 4 colour gum. They are printed at two sizes. Big and HUGE.
The first are 56x76 cm and the second 92x110 cm.
As you can imagine, working at this size has some unique difficulties.
Despite what I wrote before - I have settled on coating with hake brushes. I dip a 3 inch hake into the emulsion and drag it in lengthwise strips, dipping before each application, then once there is a complete layer, I alternate crosswise and lengthwise about 3 more times with the same brush but not adding more emulsion. This leaves me with a relatively smooth coat which I buff - archaeologist style - before it gets too tacky with another clean hake.
I give the coat a little blast with a hair dryer to help it set up then leave it for about 40 minutes to dry flat.
I measure the conditions with a handheld psychrometer before coating - I'm working at between 30% and 65% relative humidity and between 9°C and 15°C.
I sometimes get 'fisheyes' where the emulsion seems to retract through some kind of surface tension action. I currently just go over them with the emulsion brush a few times until they don't appear any more - often this is progressive - with the fisheye becoming smaller with each brush stroke. If I can't get it out at this first brushing stage then it usually goes away when buffing. I will try adding some isopropyl alcohol to the emulsion in future to see if this makes the coatings smoother and less prone to fisheyes.
Each pigment seems to create a slightly different kind of emulsion - blacks are quite gluggy and get mini fisheyes when I buff, yellow gets smooth with almost no effort and magentas seem very difficult to get brush strokes out of.
I mix convenience pigment mixtures from whole tubes of watercolour pigment with set quantities of gum. This gives me an easily measurable convenience mixture of known concentration. I mostly work with a 1:1 gum : pot.dich ratio.
I take into account the gum in the convenience mixture when mixing up a batch - for example. My yellow emulsion uses 6ml of convenience pigment mixture, 8ml of straight gum and 13.2ml of saturated pot dich. (I use syringes with pipetting needles to measure quantities - I get funny looks from the chemist employees when purchasing them - but at least thats less embarrassing than getting caught using the junkies vending machine in the red light district)
There is a great gum printer called Tony Gonzales who prints ELEVEN layer colour gums - his process is outlined in Christopher James' book. I started mixing my emulsions this way when I was testing out his way of using different ratio emulsions and it stuck even though I've started doing things my own way.
I expose under a bank of ten 4 foot blb fluoro tubes in a commercial vacuum frame which I found fortuitously after talking with the guy that makes my image setter negatives. Its all hooked up to a timer out of an old microwave and my exposures are running to about 8 minutes at the moment. For some reason, the magenta layer exposes much faster and runs about 6:30.
One day I'll make the 10 inches between the glass and the tubes about 4 inches and reap the rewards of shorter exposures - but I'm not up for more carpentry at the moment.
I do a first wash in a tank for about 5 minutes until I see the dichromate leach out and the pigment starting to run. I then place the print in another clean tank which constantly drains and is fed clean water from a float valve out of an old toilet sistern. The print stays face down, undisturbed for an hour - then I check it every 15 minutes or so until my stouffer steps are developed to where I want them. When they come out I let them run while sticking to a Perspex sheet for a bit. Once they stop running I lay them flat on a boxboard to dry and use a damp paper towel to remove any pigment that has run into the borders.
Registration in gum printing is a pain in the ....
After a certain physical size, no amount of pre-shrinking makes the paper so stable as not to change between layers. I've explored a huge number of options and I think I'm onto a good thing - but I won't let the cat out of the bag until I've perfected it. Let's just say that I do NOT want my prints mounted permanently but I want the perfection that mounting brings (a la Keith Taylor). My stubbornness is paying off however - so stay tuned.
So! Thanks for reading my ridiculously long post - and if you're in Sydney in February 2012 - come and see my show.
Wow! I would love to see those prints! I've never attempted anything bigger than 8x10. Some great ideas you've put forward about getting some control on this challenging process.
What do you use for negatives?
Originally Posted by BenjaminAustin
I'm using imagesetter negatives. They're 280 line screen halftones but considering the imperfect registration and general fuzziness of gum - you'd never know.
I tried working with a few pre press guys and finally found someone who was curious about the process. It's turned out to be pretty important to have a good relationship with the prepress guy and he's really helped me solve some tricky problems.
Imagesetter negs are also pretty hardy which is great cos I'm a bit messy and rough with them.
I calibrated from step tablets and have a very bendy curve that I wasn't sure was right at first - but seems to be pretty good so long as I include a k layer. The prints look pretty anaemic without it.
Thanks. I've printed pretty large myself - up to 92x92cm. so you have me beat! Is your work visible online someplace? I'd love to see it.