Making Custom Inkjet Negatives for Gum Printing:
Making a plain digital negative is an easy process of pressing the print button. Getting a PERFECT, personally calibrated digital negative for your own needs on the other hand can be an arduous task! I spent the better part of 2 years on and off trying to figure out how to make the best digital negatives I could for my work, often feeling more like a scientist then an artist. Inventing this process took tons of patience, but it strengthened my gum printing experience and helped me teach myself a great deal more then I had previously known. Now it takes only a click or two to load a few settings before I print my negatives, and I have a perfect calibrated negative for my printing needs. I can finally get on to printing Gums! Customizing your own inkjet negatives by following my steps should only take a few hours! By now you should have made a few gum prints to be familiar with the printing process, and have some prepared paper ready to go, no different then what you would use for printing. Everything you do while testing should be exactly the same as when you actually make real prints, for example...the time that you personally develop your prints. This is why I recommend you be familiar with the gum printing process and know your own habits before taking this section on.
Outline: A Recap on Scales Minimum Exposure time Scale Tests Making and Applying the Curve
Supplies for Calibrated Inkjet Negatives: >> Photo Quality Inkjet Printer (I have an Epson Stylus c66*) *the inks used must be able to block ultraviolet light. I used cheap inks once and found that light would go straight through those negatives, even in the darkest areas covered by 90% ink! Inks used should be resistant to fading, thus containing something to block UV light. >> Inkjet Transparencies (I use overhead transparencies from Office Depot) >> Image editing software (like Adobe Photoshop) >> An UV Exposure unit (Highly recommended for accurate testing) >> Cooking Timer with Minutes and seconds (Highly recommended for accurate testing) >> A flatbed Scanner (Highly recommended for accurate testing) >> Prepared Paper (see preparing paper)

Step One: A Recap on Scales

A brief recap on Scales from lesson 7, Making a Digital Negative:

Again, Digital images have many values of grays from black to white within them. An image containing all the values would have a full scale. Instead of thinking of a scale with the numbers 0 - 255 like a computer, we can think of the values in something more familiar: percentages. The percentage represents the amount of black. 100% is total black, 50% middle gray, 0% is white of course. The image below illustrates a scale in 2% increments with 100% 50% and 0% marked.

Your monitor should have the right contrast and brightness settings to be able to see each 2% increment distinctly.

In our computer and on our monitor, black is at 100% and white falls at 0%. If we were to print this exactly as it is onto a transparency, and use it as a negative for gum printing, we would loose a great portion of our scale in our Gum Print. Why? This is because Gum printing is a very high contrast, short-scale photographic process.

If we were to print the above scale in Gum, we would inevitably, depending on exposure time, light source, pigments used, development and a host of other variables, end up with something that looks more or less like the scale below:

(Remember, black on the negative will block out light and cause the emulsion to wash off during development, revealing the paper below, this is why 100% is now white in this scale and 0% black.)

In the above Image, we see that the high contrast of the gum caused us to loose about 2/3rds of the information on our negative and in our image! Values in our negative above 70% are washed to white (This is photographic detail literally washing off into the sink!) and below about 35% is exposed completely to a solid black. Below is a simulated example of an image loosing all of that detail in the printing process.

To combat this problem we COMPRESS that digital image scale of 0% to 100% into a range that will keep most of our image from washing off into the sink.

This is a preview of what we will be doing to the digital image's Scale:

(these numbers are approximate...I just made them up)

Explanation: from left to right

1. We start with our full scale from 0%-100% displayed here on the left in 5% increments. This scale is printed on transparency film and printed in Gum, shown here as Gum 1.
   
   
2. We loose a lot of the image detail in the dark and light grays, most of it falling into either black or white. We note that the Gum Layer becomes white at about 70%, and becomes black at about 35%.
   
   
3. We take these Numbers and plug them into Photoshop's Curves, telling it to turn all 100% black to 70% and all white to 35%. Everything in between is calculated and you now have a really low contrast compressed scale to print.
   
   
4. When you print the compressed scale, in theory (since it just happened before) the 70% should prints as white and 35% should print as black on Gum 2, your second print. We have preserved all of the details in our image scale and translated them into the gum print in an exact way!

Of course, I started with this idea 2 years ago, and found it works well in theory and in practice to a point, but unforeseen factors do come into play altering the expected results. For example, compressing the values actually does cause a loss of values. In this example, you are taking 256 individual shades of gray and squashing them into only about 90 values of gray. This causes you to still loose 2/3 of the image values, however the loss is evenly distributed rather then loosing all the high tones or low tones. I have developed ways of dealing with those factors which have proven successful. And so we shall continue on...