iso propal or everclaear

[JOSarff]

My memory has failed me. Which is the correct vehicle for thymol? Ethanol, 2-propanol (pure), 2-propanol (70%) or water?

Joe

[Hexavalent]

Isopropyl works just fine. I prefer to use 100% (or electronics grade 99.953). Thymol is insoluble in water.

[JOSarff]

Thanks Ian

[Photo Engineer]

As long as the alcohol is not denatured, either can be used.

PE

[dwross]

A tangential note: Thymol should be considered an unnecessary ingredient for almost all emulsions we're likely to make and use. It's a preservative, and basically a carry-over from the days before reliable refrigeration and when gelatin was more likely to be contaminated with odd bacteria. If you make small, frequent batches of emulsion and use them within a week or two, you won't need a preservative. I strongly recommend this for two reasons.

1) Making good emulsions is all about practice, practice, practice. If you make up big batches, infrequently, you'll likely never learn the 'tricks', which are nothing more than the application of experienced observation.

2) The characteristic curve of an emulsion gradually changes during long storage, preserved or not. Any attempts at standardizing to a contrast grade will be made much harder unless you also standardize to a pre-coating storage time range.

d

[Photo Engineer]

There are several factors to consider here:

1. I make up gelatin at 20% and 10% in advance for storage to reduce darkroom prep time. The preservative is useful here. I can keep preserved gelatin for 6 months or so and hope to increase this to a year or so soon with a wider range of preservatives.

2. Anyone who knows biology, knows that gelatin is a good growth medium for all sorts of micro-organisms. We are warned not to work with our emulsions and gelatin solutions if we have a cough or strep or any form of illness like that. This can "turn" an emulsion or gelatin just about overnight.

3. The experiments that I showed at several of my workshops were from emulsions up to 6 months old and the coatings were over 1 year old. Curve shape looked pretty good to me. I could not have kept the emulsions for 6 months or more without thymol.

4. I make my standard emulsions in 120 - 500 gram batches and often mix them, they are so much alike. I keep a 1 L can of the blends for my common coating checks to refer back to when I make experiments. If I don't know where I have been, I don't know where I am going!

5. Finally, use of Thymol or any preservative is useful, as you then have a coating formula which includes this chemical and therefore does not need a big change when you change workflow to included older emulsions and gelatin solutions.

This all summarizes why I teach the use of preservatives in my workshops!

PE

[Hexavalent]

I would tend to think that preservative not only serves a purpose in 'wet' emulsion, but also adds a measure of protection to the finished, dried product. Better safe than sorry AFAIC. Besides, that certain Thymol aroma is one of the darkroom fragrances that tells me I'm doing something

[Photo Engineer]

Well, the Thymol eventually evaporates out of the finished, dry coating, but it can help. It is present in packaged sheets and the preservative is certainly present in commercial products. If you open a package of Ilford paper, you can smell the phenol which is what is used as a preservative in their B&W papers. That is so strong and toxic when used in the home darkroom that I don't suggest it, but it is used by Ilford and Schoeller as well in large scale operations. Kodak does not use either Thymol or Phenol. They used equally potent materials. I'll comment on them soon.

However, there is another important fact involved in this that I did not mention earlier.

All batches of fine photo grade gelatins are packed with an assay, just like fine chemicals. Only in this case, it is a bioassay that includes things such as tuberculin bacilli, staph, strep, E. Coli and etc per unit weight. This means that the dry gelatin contains a certain amount (printed on the assay), per specified unit of dry gelatin. Well, when you mix your gelatin in warm water those microorganisms begin to multiply unless you stop them. I don't know about you, but I don't like handling gelatin without some sort of preservative there to retard their possible growth!

PE

[Hexavalent]

As far as microbes are concerned: "there's always room for Jello!"

[Vaughn]

And us carbon printers toss in a bunch of sugar, too!

Does the alcohol itself help keep the beasties away (I use about 10ml of 100% isopropyl per 750 ml of glop)?

And does the exposure time under the UV lights (even tho it is not the deadlier UVC) help kill the beasties?