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Photo Engineer
12-25-2008, 08:37 PM
I have commented before on the fact that old emulsions used active gelatin and therefore are not truly usable without some sort of conversion with modern inactive gelatin.

Here is a handy hint that is different from using sulfur or sulfur + gold to get the same results as the old timers.

I've equated temperature with speed. Up in temp, up in speed. I've also equated addition time with contrast (and to an extent speed) in that up in addition rate, up in contrast.

These two general rules can allow one to convert an emulsion from the old style to the new style with minimum pain and still use modern gelatins. The trick is to raise the temperatures and decrease the addition times used in a formula.

Here is an example. Let us assume a formula uses 5' addition of Silver Nitrate at 40 deg C and then a digest of 20 minutes at 50 degrees C. To convert, a hint would be to add the Silver Nitrate over 2.5' at 50 deg C and digest 20 minutes at 60 degrees C. This is just a hint of a starting point, but you get the picture that time and temperature can be used to "adjust" a formula for use today and it is possible to omit the sulfur sensitization step otherwise needed for increasing speed and contrast when using refined gelatins.

Just a handy tip. You will have to play with your formula to get what you want out of it, but here is how you start.

PE

wildbillbugman
01-07-2009, 06:54 PM
Hi P.E.,
Do you have an idea as to what the upper limit to temperature might be? How high can you go until the rule of 'higher temp. while adding Ag increases speed' is no longer valid. I have read that some emulsions go as high as 70 degrees.
Regards,
Bill

Photo Engineer
01-07-2009, 07:17 PM
Bill, some emulsions go up as high as 95 deg C. These were "boiled emulsions" from the old days. Gelatin begins to denature at that temperature, and the mess is near to boiling. But, the exact answer is that the temperature can go as high as needed but must not boil or denature the gelatin. Generally, this is about 80 - 85 C.

PE

rmazzullo
01-07-2009, 09:13 PM
Hello PE,

With this in mind, as well as emulsion making in general, is there a "tolerance" that is allowable (plus or minus some amount) when measuring temperature, and keeping kettles / solutions at a desired temperature for the required length of time?

I know I can spend money for very accurate temperature measuring and / or control using thermocouples, feedback loops. PCs, heating jackets, etc , but could good results be obtained with a "lower grade" of equipment repeatedly?

(A friend has a large roll of thin nichrome ribbon that would be ideal for making heating jackets for kettles, etc., but I suspect this is venturing far into the region of overkill. Regardless, I have yet to convince him that he has no real use for it.)

Thanks,

Bob M.

Photo Engineer
01-07-2009, 09:24 PM
There is no fixed time period for how long gelatin can be kept at high temperature. The degradation is either slow or fast depending on gelatin type. If you can, keeping track of viscosity might help as that is a primary indication of the degradation process. But why worry? We know that 1 hour at 60 C is good for these emulsions, so that is a good center point to work from.

PE

Kirk Keyes
01-08-2009, 12:19 AM
Bob, I can assure you that good results can be had with very common and simple tools for heating and temperature control. Denise Ross makes truly fine emulsions with waterbaths and inexpensive thermometers. I've seen PE use a hotplate with thermocouple control, and that works too, but it's really not as fancy as you are asking about.

Try some of the digital cooking thermometers that you get at your local kitchen shop. They run about $25 and are certainly good enough.

rmazzullo
01-08-2009, 11:40 AM
Kirk,

Thanks for the reply. I have no doubt good results can be obtained using simpler methods and tools. But if I just happen to make a good emulsion one particular day using common items for heating and temperature control (among other things), there's no guarantee that I can repeat that particular success if variables exist in the tools I use.

I am willing to bet that I could probably have better luck making a more consistent emulsion, if I use better, more precise tools. At the very least, I will have a much better database of the mistakes I make when I know and can record what exact temps, addition rates, etc were involved. I am willing to go the extra mile to remove whatever uncertainties I can. The less I have to guess, the better.

(edit) Right now, the only area that I would use items that I could get from a hardware store, or a shop that sold kitchen items is in setting the emulsion after coating and drying. I don't think the need for precision is as great.

Bob M.

Kirk Keyes
01-08-2009, 03:03 PM
Bob - look for a new thread on temp recording.