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Gum Bichromate

Brief Overview

Gum Bichromate is a unique 19th century photographic printing process and it was also the first ever colour photographic printing process. It is possible to print any colour you desire and to print multiple layers to create a print with a large tonal range. Edward Steichen's gum print the pond, moonlight is the most expensive photographic print ever to sell at auction. It is suggested that the soft painterly characteristic of gum is the result of the Steichen success.

How does Gum Bichromate work? It is very simple: watercolour pigment is mixed into clear gum Arabic to give it colour. Then the gum / pigment solution is mixed with a solution of sodium, potassium or ammonium dichromate, the dichromate makes the mixture light sensitive. This is then coated on a sheet of paper and left to dry in a dark place. The dry gum Bichromate paper is covered with a negative image then exposure to UV light, where the dichromate hardens the gum Arabic. The paper is placed in a still bath of tap water. After a few minute the unexposed gum washes away and the hardened exposed gum stay on the paper. After an hour in the water the paper print is hung to dry. When the print is dry the gum is permanently harden to the paper, if the prints is re washed water cannot re penetrate the hardened gum on the print.

Ingredients

• • Acid free paper

• • 2 Hake brushes

• • Shot glass

• • 2 syringes

• • Sticky tape

• • Ruler

• • Normal paint brush

• • Pencil

Chemicals

• • Gum Arabic

• • 25%Potassium Dichromate or 10%Ammonium Dichromate solution

• • Water colour pigment

Note: Dichromate is very harmful to your health and is carcinogenic. It should be handled with care, always wear gloves when using it wet or dry. If you mix it from a powdered state always wear a dust mask. If you get it on your clothes remove them straight way and if you get it on your skin wash it off.

Water colour pigment

It is possible to print using any water colour pigment you choose. Winsor and Newton are the best pigments for gum printing but a cheaper brand is Art spectrums, this works fine. I use the following pigments below:

Monotone (B/W)

• • Lamp Black (PBk6)

3 Colours

• • Magenta (PV19)

• • Yellow (PY37)

• • Cyan (PB29)

4 Colours

• • Magenta (PV19)

• • Yellow (PY37)

• • Cyan (PB29)

• • Lamp Black (PBk6)

Paper Gelatin Sizing (Optional)

Sizing your paper with gelatin gives you a better surface to printing on, it stops the paper from soaking up the emulsion and will produce more contrast and a much sharper print. Sizing also allows you to wash the paper multiple times without the paper breaking down.

Warning: Formaldehyde gives off harmful vapors, so when handling it wear a gas mask and use it in a well ventilated area.

Ingredients

• • Acid free paper

• • Gelatin

• • Distilled Water

• • 5% Formaldehyde

Note: Food grade gelatin will work but photo grade gelatin is better. Formaldehyde is reusable, the amount needed is just enough in a bath to cover the paper completely.

Pre mix gelatin:(3% gelatin mix) Heat up 1000ml of distilled water to no more than 40c, Mix in 30g of gelatin and allow to sit overnight or until the gelatin crystals have dissolved completely.

Step 1: Select the paper you would like to use. Thick paper handles better than thin paper that can fold or rip when it is wet. The smoother the paper the flatter and closer the negative will sit to the light sensitive emulsion this will make the image sharper.

Step 2: Stretch the paper by placing it in a bath of hot tap water for 20-30mins, then hang out to dry.

Step 3: Reheat the gelatin mixture slowly in its container in a bath of hot water (not boiling hot water) until the gelatin turns into a liquid state ( don't let the gelatin exceed 40c or it won't set again). Place the liquid gelatin into a bath that is sitting in a bigger bath of hot water to keep the gelatin warm. Now the gelatin is ready for sizing.

Step 4: Place the paper into the gelatin bath for 20-30mins.Then remove the paper from the bath and squeegee the excess gelatin off the paper then hang it out to dry.

Step 5: Place the dry gelatin coated paper into a bath of 5% formaldehyde for 20-30mins.

Step 6: Remove the paper from the formaldehyde and place into a running water wash bath for 30-60mins

Then hang to out to dry and when the paper is completely dry it is ready to use.

Pigment test

Before starting printing, a pigment test is needed to check the density of the pigment to gum Arabic ratio. First, select the paper you will be printing on, cut two small strips of the paper to do your test on. Down one side write in pencil 1ml through to 10ml, this will indicate the ratio of gum Arabic to pigment used in the test. Write on the top of one of the strips 'wet pigment test' and the other 'dry pigment test'. Then measure out 10mm of pigment in a shot glass, use a rule to be as accurate as possible, this is just a starting measurement, you may need more or less pigment. Take the shot glass of pigment and mix in 1ml of gum Arabic using a normal paint brush, once the pigment and the gum is combined brush a small stroke of the gum/pigment next to the 1ml on each of the dry and wet pigment test strips. After that add another 1ml of gum Arabic to your mixture, stir together and put a small stroke of gum /pigment next the 2ml marks. Repeat until you reach 10mls. Let the pigment test dry and then put the 'wet pigment' test in a bath of tap water for 1hour. After an hour remove the wet test and let it dry. The wet test indicates how much pigment will wash out during the printing process and represents the unexposed, unhardened gum arabic. The 'dry test' shows how dense the gum pigment will be when the gum is exposed and hardened. What you are looking for on the wet and dry tests is a slight visible stain on the wet test and a dense non transcendent mark on the dry test because this will be your 'blacks' in the final image. See the test below for example of what you are looking for.

Mix up a working stock solution of gum and pigment-

for example if your pigment test told you '1mm of pigment to 3ml of gum Arabic', mix up 5 times more solute, making it '5mm of pigment to 15 ml of gum Arabic'. The quantity of stock solution is up to you

Printing

Step 1: Tape the paper to a work table so it cannot move during coating.

Step 2: Mark out film position on the paper using a pencil, this will let you know where to coat the solution on the paper. If you are doing multiple layers, register your negative or negatives by pinning all 4 corners of the negative to you paper, this will allow you to line up accurately the negative again for another coat or layer.

Step 3: mix 1 part dichromate solution to 3 or 4 parts stock gum/pigment in a shot glass (only make up what you need for one coat at a time). Coat paper with the Gum Dichromate solution using a hake brush, use a second brush to smooth out or remove any uneven Gum Dichromate solution. If you are doing a multilayer monotone gum use a 1 dichromate to 1 or 2 parts gum/ pigment solution

Dichromate to gum/ pigment ratio	1 to 1	1 to 2	1 to 3	1 to 4
Layer type	Multi layer Monotone	Multi layer Monotone	Single Layer Monotone	Single Layer Monotone
contrast	low	high	low	high
Tonal range	long	short	long	short

Step 4: Dry the Gum Dichromate solution by using a hairdryer or by placing it in a dark area until dry.

Step 5: Tape down the negative over the dry Gum Dichromate, If you are doing a second layer re-register the negative using the pinholes. Once the negative is registered using the pins, then tape the negative down and remove the pins.

Step 6: Expose Gum Dichromate in a UV unit under a heavy piece of glass or in a contact frame in the sun. Do a test strip to judge the exposure time.

Step 7: Then wash.

Wash

Bath 1: place the gum print face down in a bath of tap water, do not touch the image because it will rub off, ( 20mins)

Bath 2: place the gum print face down in another bath of tap water, do not touch the image because it will rub off ( 40mins)

Dry.

Once the print is dry the image will not come off if you touch it, now it is ready for another layer if you are doing multiple layers

Multiple layers

Gum Bichromate is famous for its ability to do multiple layers, the multiple layers allows you to make 3D images, mono tone images with large tonal ranges and 3 colour and 4 colour photographic prints. Always start with the lightest shade or coloured layer first and end with the darkest. Below is a chart showing you how long each layer needs to be exposed for in a multi layered print. All the print types below should have the overall exposure time. So if you do a single monotone print you can use the overall exposure time to judge the expose for any multi layer gum print. For example: if the overall exposure time for a single layer monotype is 1200 units in a UV unit a 4 colour gum would have a layer of yellow at 300 unit, a magenta layer at 300 unit, a cyan layer at 300 units and a black layer at 300 units making to total time your print spent in the UV unit was 1200 units.

Multiple layers	1^st layer	2^nd layer	3^rd layer	4^th layer	Overall Exposure time
Single layer Monotone	Colour 100%				100%
Multi layer Monotone	Colour highlights 50%	Colour midtones 33.33%	Colour shadows 16.66%		100%
3 Colour Gum (RGB)	Yellow 33.33%	Magenta 33.33%	Cyan 33.33%		100%
4 Colour Gum (CMYK)	Yellow 25%	Magenta 25%	Cyan 25%	Black 25%	100%
3D Gum	Red 50%	Blue 50%			100%
3D Gum with Shadow layer	Red 33.33%	Blue 33.33%	Shadow 33.33%		100%

How to make a Digital Negatives

Monotone Negatives

Using Photoshop

Select your image: open it in Photoshop

Convert to Black and White: Image / Mode/ Greyscale

Check the blacks are 100% black: side tool/ drop tool/colour sampler tool (cst) / use cst to select the darkest area on your image / use curves to make the darkest area 100% black (image/ adjustment/ curves/ adjust the curves as needed/ok)

Check the whites are 100% white: side tool/ drop tool/colour sampler tool (cst) / use cst to select the lightest area on your image / use curves to make the lightest area 0% white (image/ adjustment/ curves adjust the curves as needed/ok)

Flip image horizontal: image/ image rotation/ flip canvas horizontal.(any text in the image should read backwards)

Invert image to a negative: Image/ adjustment/ invert or hold command and I on the keyboard

Convert to 8bit file: Image/Mode/8 Bit

Save as: Tiff for inkjet printers and PDF for laser jet printers

Print: Print on acetate

3 Colour Gum Negatives

Using Photoshop

Select your colour image: open it in Photoshop

Flip image horizontal: image/ image rotation/ flip canvas horizontal. (any text in the image should read backwards)

Convert image to RGB colour file: image /mode /RGB colour.

Split colour channels: layer tool bar/ select the channels file / then to the right of the channel file (and paths file) is a little square click the square/ split channels

When the channel split they will make 3 separate black and white documents titled Red, Green and Blue. Rename the layers - Red is Cyan, Green is Magenta and Blue is now Yellow. The renaming of negatives corresponded to the pigment you will use for each gum layer.

Invert each channel to a negative: Image/ adjustment/ invert or hold command and I on the keyboard

Convert to 8bit file: Image/Mode/8 Bit

Save each channel as: Tiff for inkjet printers and PDF for laser jet printers

Print: Print on acetate

4 Colour Gum Negatives

Using Photoshop

Select your colour image: open it in Photoshop

Flip image horizontal: image/ image rotation/ flip canvus horizontal. (any text in the image should read backwards)

Convert image to CMYK colour file: image /mode /CMYK colour.

Split colour channels: layer tool bar/ select the channels file / then to the right of the channel file (and paths file) is a little square click the square/ split channels

When the channel split they will make 4 separate black and white images titled Cyan, Magenta, Cyan and Black. The new images corresponded to the pigment you will use for each gum layer.

Invert each channel to a negative: Image/ adjustment/ invert or hold command and I on the keyboard

Convert to 8bit file: Image/Mode/8 Bit

Save each channel as: Tiff for inkjet printers and PDF for laser jet printers

Print: Print on acetate