Hello and welcome to APUG!
There is a small community of people here who practice silver gelatin emulsion making and coating, and there's
a whole forum on the site dedicated to it! There's a lot of good reading there, maybe you've already come across it. I've actually suggested to the mods that we move this thread over there so it is more visible to the appropriate users of this site.
Simple silver bromide emulsions that can easily be made in the home darkroom are mostly sensitive in the UV, and also a bit into the blue. If you look at old photographs which used emulsions which were not spectrally sensitized, you will see that the sky appears white, and there are never any clouds visible. This is because of the large amount of blue and UV light in the sky to which the emulsion is sensitive. Often landscape photographers like to have defined clouds visible in the sky, so having a UV/blue sensitive emulsion is a big limitation. Likewise a UV/blue sensitive emulsion is limiting to portrait photographers. Skin blemishes (and also skin features like freckles
) absorb in the UV/blue. This will show up as dark spots on the skin which many find unflattering in a portrait.
Adding a sensitizing dye will allow the black and white emulsion to absorb longer wavelengths of light. If you add a dye that sensitizes for green, the emulsion is called orthochromatic. If you add dyes that sensitize for green and red, the emulsion is called panchromatic. Some specialty films extend the emulsion's sensitivity into the near infrared, beyond what human eyes can see. Adding these sensitizing dyes will not create a colour emulsion, the photo will still be black and white. Rather, adding sensitizing dyes allows you to record detail of objects of the colour which the dye sensitizes for. For example, using an non-spectrally sensitized emulsion to take a photo of a red jacket, the jacket would appear black in the print, without much detail. If you instead used an emulsion which had a red sensitizing dye added to it, the jacket would not be black, and it would show considerable more detail. Hopefully I made this less confusing rather than more!
The sad news as that most "proper" sensitizing dyes are ludicrously expensive and have poor shelf life. The good news is that you can use erythrosine (FD&C red No. 3) which is a common food colouring. This is a green (or ortho) sensitizing dye, but it is pink-red in colour. Although initially unintuitive, this is common for sensitizing dyes. Remember, the molecule is absorbing green light (removing it), so the colour of the molecule (i.e. the colour of the light reflecting off the molecule which it does not absorb) appears opposite to green on the colour wheel: that is, red. Unlike "proper" sensitizing dyes, erythrosine should be added before precipitation for best results, although it can be added afterwards as well. Even with erythrosine, if you want to get an "orthochromatic look" to your photos, you may have to use a yellow filter to cut down some of the blue light so that there is relatively more green light exposing the emulsion.
If you are really interested in emulsion making, I highly recommend trying to pick up a copy of a book written by a retired Kodak photo engineer who is active on this site. The book is called "Photographic Emulsion Making Coating and Testing" and is usually available from the
Photographer's Formulary and
Fotoimpex (although they both seem to be sold out at the moment). There is also Dennis Ross' excellent website, The Light Farm which explains much of what I wrote above, but using photos as well. It's a great site,
try starting here for some info about erythrosine and making orthochromatic emulsions.
Best of luck, hopefully this wasn't too intimidating! Feel free to ask for clarification.
P.S. At the risk of over complicating things, I will say that if you are interested in capturing colour images, you will need to make a panchromatic emulsion (both green and red sensitizing dyes added) and then take three photos, one with each Red #25, Blue #47 and Green #58 filters. You now have the three RGB channels of the colour image and you can combine them in various ways to be able to see the full colour image. See Denise's results of
this technique here.
Finally, if you are just wanting a monochrome image that is coloured, you can try out
toning or alternative processes like cyanotype. Hand colouring is also a possibility.
