The present paper aims to be a reference guide to anyone in the EU willing to go on printing with pigmented gum while being fully compliant with the actual safety regulations regarding chemical substances (i.e. avoid the use of dichromate, illegal as of September 2017 in the EU).
It is well known to anyone involved in historical/alternative photographic processes that, unlike in the USA, dichromates have been banned in the whole European territory as of September 2017.
Although many people may have some leftovers of the sensitizing agent still in their cabinets, it’s hard to imagine that anyone would go on using illegal substances even in small quantities, also because health concerns have more than solid background and vast scientific literature is available documenting how hazardous the hexavalent chromium compounds can be.
Research on different formulation on photoresist composition is not new, as obvious. Especially in the graphic arts industry leading companies has put significant amount of resources in experimenting and patenting formulation based on compounds other than dichromate as a mean to deliver faithful reproduction of color-separated images1. Most of this research dates some decades ago but the insight they provide are very useful for our purposes.
As shown by the referred documents among many others of the kind, the chemistry behind printing industry is not a simple matter nor it is merely transferrable with basic tricks in a home environment where chemistry handling and safety procedures have obvious limitations.
While in the printing industry the ban of chromium compounds has no longer much significance as printing process are mainly performed via inkjet technology with much safer substances than dichromates, chromium compounds still has a role in special application involving the creation of a photo resist matrix.
Following the CR(VI) ban some well known photosensitive compounds have been targeted as chromium substitute also by large companies actively seeking a way to replace PVA-dichromate resists in the metal – forming industrial process. Namely, Veco BV, a large dutch company that manufactures nickel metal forming mandrels for a variety of applications, conducted a vast screening of different alternatives and finally filed an authorization request to the ECHA to continue to use dichromates because the alternatives were not suitable for that process2. Nevertheless the same reasons do not apply to our field, as we do not need such a strong adhesion of the hardened matrix to the substrate.
The best candidate compound is known as DSR or Diazo3,a compound which is already used in screenprinting to prepare the sensitized screens.
The goal of our research is to test whether and how use Diazo compound to print our images in our small labs, without coping with any industrial process and avoiding other hazardous chemicals which are, in the cited papers, sometimes involved in the process.
This possibility was confirmed by Mr. Travis Pugh, who in 2014 started his own research with the Diazo compound in gum printing with very interesting results.
So, a small group of people, pivoting on LaboldTech expertise in chemistry, begun experimenting and we still are at present time.
Although our research is far from being completed, we would like to share some interesting results which may be of interests for European photographers to go on with the practice as well as photographers from other countries who may want to work in a safer environment ceasing to handle very hazardous chemicals.
2) Summary of findings
A suitable sensitizer can be obtained by making a 0,5% solution of Diazo with distilled water. Since the shelf life of the solution is limited (less than one week, I limit its use to 48 hours) a double decimal scale may prove necessary to accurately weigh very small amount of the powder, in the range of less than 0,1g.
The sensitizer: gum ratio is the same as with the dichromate, roughly 1:1 as a rule of thumb.
Although best sensitivity is in the range of the long UVA, namely 380nm, usable lamps include high pressure sun tanning lamps, black fluorescent tubes and plant growing lamps.
Speed and contrast
Speed is approximately one to two steps faster than dichromate. No contrast adjustment can be made by varying sensitizer concentration, as the working range of the Diazo is much more limited. So, to avoid fussing around with hundreds of tests, it’s far easier to adjust the contrast of the negative by refining your curve.
Contrast seems to be higher than dichromate, but I suppose this can be related to its more limited range of effectiveness. Thickness of your layer as well as pigment density affects speed just as it does in gum dichromate, so experimenting with your own pigments is necessary to establish a sound workflow.
This is one crucial aspect of the process. Gelatin sized papers are usable but because Diazo stains the gelatin, a clearing bath has to be used afterwards. It seems preferable using PVA, as the “in-progress” print colors will much more closely resemble the final print. Current tests show that a bit more sizing is required in comparison to dichromate. Using two layers of Gamblin PVA Size with distilled water (first 1+2, second 1+4) on Fabriano Artistico paper seems to work best, while only one 1+2 layer was requested with dichromate.
Based on our results, sizing seems to be the most critical issue when it comes to clearing the highlights. A small amount of pigment, easily removed by a very light water spray, is often visible in the highlights.
Diazo Stain Clearing
After development, a short (30 seconds to one minute) exposure to UV clears most of the residual yellowish stain on PVA sized papers. The re-exposure seems to affect gelatine sized papers much less than those sized with PVA, and further test have to be made in order to assess if the persistent stain is due to gelatin itself or the hardening agent used4 or to other factors5. As reported below, a clearing bath can be used to remove the stain from the print after completion.
3) Diazo Process in detail – our experience
The process does not differ much from dichromate based printing.
By keeping in mind at each stage that diazo compounds are heat sensitive, a best practice could be to store the diazo powder in the freezer while keeping the solution in an opaque bottle in the refrigerator.
Try to mix up what you need for, let’s say, two days to a week. Some literature about screen printing claims that diazo based compounds have a shelf life of some months. It does not seem to apply to our case, although more tests can be performed to assess this aspect.
We also suggest to try and use the same batch for at least the basic three (or four) layers in a print, to minimize the effects of differences in the “basic structure” of the print.
We suggest using a double digit scale, able to weigh 1/100 of gram, as diazo must be precisely dosed. The sensitizer solution is 0,5%, so to make let’s say 100 ml you only need 0,5grams of powder. Cheap ones are available on major online retailers. If you use a single digit scale the possible error can play a significant role in your solution concentration and compels you to mix up a large quantity to minimize the error.
A good practice could be to keep a small portable refrigerator close to the darkroom, to conveniently store both the powder and the solution during the print session.
Everything must be done under safelight. Even though we try to work at this stage under red safelight to minimize powder exposure to light, we found that a common 15w tungsten bulb at two meter distance can be used as safelight.
We prepare the amount of gum/pigment mix needed in a glass beaker, then we take the sensitizer from the fridge, take out the amount needed with a syringe and store it back. Try to avoid keeping the sensitizers, both powder and mixed, at room temperature by putting them back in the refrigerator a.s.a.p.
Coating is made by weighing an equal amount of gum/pigment mix and sensitizer. As a rule of thumb, for a 40x60cm print we use 12,5g of pigment mix and 12,5g of sensitizer. If measured by volume, this would be 12,5 cc of sensitizer and roughly 11,4cc of gum/pigment mix, as specific gravity of gum is a little higher.
Gum: sensitizer ratio is 1:1 as a basic rule and a starting point.
We found no differences in the smoothness of coating between dichromate and Diazo.
We dry the coated paper with a hair dryer with no heat. Usually, a couple of minutes is enough. Remember: Diazo is heat sensitive.
We use a variety of sources. Simone currently uses an old offset plate burner with a vacuum frame. The lamp is an high pressure suntanning UV lamp (1200watts, used at half power). His exposure times are in the range of 120 to 240 seconds. Cyan is as usual the fastest color, followed by magenta and yellow.
Black light fluorescent tubes can be used as well, as well as suntanning fluorescent tubes. Salvatore uses suntanning fluorescent tubes, with exposure times in the range of 3 minutes as well. Again, keep in mind that diazo is roughly one to two steps faster than dichromate.
Basic exposure times, as usual, must be established to match your own equipment. Please be aware that, according to the producer, Diazo has a peak of sensitivity at 380 nm wavelength, this could be useful in choosing your lights should you have to make your own UV unit.
We develop the prints face down with no agitation and we found development times to be much faster than dichromate, as maybe the gum bond produced by Diazo is weaker. This must be taken into account especially if you plan to work with thicker layers, as it seems more fragile and prone to flake off. Thin layers seem to be much less affected. A tentative development time can be 15 minutes or even as short as 5 minutes. The weaker bond allows a great deal of flexibility and it seems very responding to spray development to clear the highlights.
Quite often, when you look at the just-developed print, you will find that a yellowish stain is present in the highlights thru mid-tones, while it is not in the shadows. Although this is a known issue in the screenprinting industry, there are some workarounds to reduce up to completely eliminate the stain.
Stain appears to be due to two factors: diazo reacting with the paper and unexposed diazo.
As per the interaction with the paper nothing can be done until the print is finished, a great deal of the stain can be removed by re-exposing the print to UV right after water development, while still wet.
We found out to be very useful to re-expose the print for a short period of time (30 sec to one minute) as it seems to bring the Diazo UV reaction to completion clearing the stain.
Further chemical assessment is needed because it does not happen all the time. As usual with Alt proc, many variables come into play, mainly paper and size.
Wash the print by gently agitating it in a water bath for two to five minutes. The “in between” washing times vary according to how delicate the last printed layers are. If you feel that that particular layer is fragile and further manipulation can ruin it, just give a quick gentle rinse and let the print dry and give it a final wash after completion.
Clearing and final washing
The Diazo stain can be removed with a diluted bath, made up by mixing the two stock solutions given below6:
Stock Clearing Bath A:
- Water (at 44C°) 930 ml
- Potassium Permanganate 6 g
- Sodium Chloride (Table Salt) 14 g
- Glacial Acetic Acid 25 ml
- Water to make 1 liter
Stock Clearing Bath B:
- Water (at 44C°) 940 ml
- Sodium (Meta) Bisulfite 30 g
- Sodium Sulfite 30 g
- Water to make 1 liter
To make working solutions of A and B, dilute each in cold water at 1:20.
Place the print for 1 minute in bath A, then for 1 minute in bath B. Please be aware that the print should be inspected closely as the clearing bath shows a tendency to reduce density in the shadows by removing some of the hardened gum.
Final wash is 20-minute in tap water.
Since it is known in the screenprinting industry that a persistent Diazo stain can only be removed by a Sodium Hypochlorite-based bath, further tests are to be performed in order to establish if we can achieve enough hardness in the crosslinking phase able to withstand the hypochlorite bleach phase. This could be attained either in the exposing phase of the process by adding other chemicals (i.e. acid – crosslinking monomer) or after the process is complete with a non staining crosslinking agent. No tests have been made in such respect.
4) A possible variation
A possible variation of the process can be explored by substituting polyvinyl alcohol (PVA) for gum Arabic.
This is very interesting to take advantage of new technology paint formulation. Just to make an example, using specially formulated watercolours could be interesting for they have some properties which may be appealing, like the Lascaux Sirius primary system. If mixed with gum Arabic, though, they lose much of their strength, while the latter does not happen when mixed with PVA.
Only preliminary tests have been performed on the matter.
Diazo compounds can harden PVA, no question on that. Specific tests have to be made on the subject to establish a sound workflow, especially because the molecular weight of PVA affects the emulsion speed as well as other factors.
So far, best results with the PVA show that
- PVA (polyvinyl ACETATE) sizing is required (Gamblin PVA, for example)
- still development is somewhat difficult . The good news are that spray development seems to work nicely, because it tends to be less susceptible to complete wash off giving you a good space to fine tune your intervention
- contrast seems to be higher than gum arabic
- removal of the yellow stain seems easier than using gum arabic.
We made some tests using a 7.5% PVA solution as a working base, and stepping up by further diluting it with distilled water to attain 5%, 3.5%, 2.5%.
Oddly enough, some good results have been possible using a very different combination of sensitizer concentration with respect to the polymer.
Best results so far has been reached by working with full strength 7.5% solution and lowering the Diazo ratio to 1:3 (i.e, for example, 1grams diazo to 3 grams of PVA-Pigment mix) and by using a double coat of Gamblin PVA Size 1+2.
Another test, also promising, was made with a different batch of PVA, raising the Diazo to 1,5:1 (3 gr diazo over 2gr of mix gum/pigment).
The difference is fairly easily explained. Working with polysaccharides one should establish a standard not only in solution concentration, but also with the molecular weight of the substance, as it affects the viscosity of the solution. Since the hardening power of the sensitizer is directly related to the ratio between the two dry weight (i.e. sensitizer powder vs polymer powder), if you choose a high molecular weight polymer you most likely will have to dilute more to obtain a suitable viscosity. In this case you should also lower the concentration of the diazo to avoid stain and blocking of the shadows. So, as a rule of thumb, we suggest testing with low molecular weight PVA.
5) Some Prints
Please let us know your experience with the process.
- Please ref. to, for example, USPatent 3721557 – METHOD FOR TRANSFERRING COLORED IMAGE AND LIGHT SENSITIVE TRANSFER SHEETS and 4960671 – STABILIZED PHOTOSENSITIVESCREEN PRINTING DISPERSION COMPOSITION WITH DIAZOCONDENSATE, POLYVINYL ALCOHOL, AND COPOLYMER OF VINYL ACETATE AND N-METHYLOC ACRYLAMIDE
- ECHA/RAC/SEAC: Opinion N° AFA-O-0000006524-74-02/F
- Our current tests are made with Benzenediazonium, 4-(phenylamino)-, sulfate (1:1), polymer with formaldehyde (CAS 41432-19-3)
- With dichromate we commonly use a single layer of 3% gelatine premixed with 2% Glutharaldheyde in a ratio of 7,5cc/liter brushed on paper. Maybe a formaldehyde or alum hardened gelatin may show a different behavior.
- We refer especially to alkaline buffer in the paper. Since stain removal seems to be easier with stronger (thus more isolating from the substrate) size and on papers, such as Magnani Revere Platinum, that does not contain calcium carbonate buffering.
- Formula kindly provided by Travis Pugh, also used in the UltraStable color process.