CAS 2718-90-3 'diazostilbene' or 'DAS' is a relatively difficult to source chemical, especially economically so. I recently ordered some from phototypie.fr at what I consider a fairly outrageous price, but still competitive if I compare with other western sources that publish pricing information online.
There must be a cheaper way to get this in somewhat larger quantities (let's say 1kg or more), but I haven't taken the time yet to track down a viable source. It's on my to-do list, but I'm first going to experiment a bit with the small quantity I've got now to see if it's worthwhile.
Note that there are some other AZO compounds that accomplish the same and are potentially cheaper, as they're for instance used for screen printing. The major drawback appears to be that those generally suffer to some extent from the same 'dark reaction' that dichromate does. I.e. the carbon tissue fogs as it ages, unless stored frozen. Still, such diazo compounds apparently produce a more stable carbon tissue than dichromate, so it may be a viable option if the tissue is made in small batches and used up with let's say a few weeks' time. I'm very seriously considering this route due to the difficult supply situation of DAS, but when checking recently my local shops were all out of AZO screen printing sensitizer...
There's one more option, at least for monochrome work, which is the so-called 'Chiba system', or at least an approach exploiting the colloid-hardening properties of ferric ammonium citrate, the same compound used for cyanotypes, which is easy to source and cheap. The main disadvantage is that it needs (1) a developer to perform the hardening action, but a simple drugstore 3% hydrogen peroxide solution works fine for this, and (2) it only works in absence of oxygen. In the Chiba system, this is achieved by overlaying an agar-agar membrane over the carbon tissue, but for tissues larger than a few centimeters, this seems to be highly unpractical. The approach Sandy King has been using for a few years and with good success, is to expose the tissue through the base. This requires the tissues to be poured onto a transparent base, which furthermore needs to allow the tissue to adhere in its unused state and during warm-water bath washing, but also allow the image to transfer readily to its final support.
I did a quick experiment the other day with some ferric ammonium citrate tissues poured onto albumen-subbed laser printer transparencies and while they seem to expose just fine, they wash off the tissue support in the hot water bath. I suspect the bond that the hardened gelatin creates initially is just not strong enough to keep the relatively heavy gelatin image in place, and/or the albumen subbing is destabilized by the hot water bath. Indeed, Sandy himself has noted that his best success has been with materials that are already subbed industrially to take gelatin emulsions, i.e. materials like fixed out lith film and imagesetter film. I have not tried any of these yet. Xray film should also work and might be a more cost-effective and easy to obtain alternative, although imagesetter film when bought in roll quantities shouldn't break the bank either.
So long story short, there are alternatives to DAS and dichromate, but they all have their pros and cons. I suppose this is why DAS always ends up 'floating on top' of peoples' lists, because despite its price and poor availability, it is technically at least so far the superior option.
Interestingly, the several threads with people suggesting they might try other AZO sensitizers are never followed up with actual experiences, and all we have in that direction (including compounds like the Diazon 22LZ azo compound mentioned by King et al.) all track down to the same 2018 book by King, Nelson & Lockhart where it's stated that these compounds work, but sadly without illustrations or further details.
www.scbt.com
) I'll give it a shot. I gave up on the idea of this being an affordable experiment some time ago.
