That's somewhat surprising, given TiO2 shows significant light activity (photocatalyst, solar cells etc.) under UV radiation.
Originally Posted by Photo Engineer
Yes I was wondering about that myself. It has long been known that TiO2 is sensitive to light and that there was trouble from it in early RC papers... I take it that they found an ingenious if not perfect work around, but it sounds like a bad approach,
Originally Posted by Hologram
Sort of like using the brain of a violent criminal for brain transplant experiments....
(BTW- What was the problem with using Baryta for everything?)
Ray, auf der Frankinsteiner Platz F/a.M.
Last edited by Ray Rogers; 02-20-2009 at 09:57 AM. Click to view previous post history.
Titanox enhances the cracking and crazing of polyethylene in RC support. Use of free radical chain stoppers apparently fixes that problem. AFAIK, Titanox can be added to emulsions directly with no overt effect except that it does block some light and therefore decrease speed. OTOH, it increases apparent contrast by appearing to lower dmin when viewed by reflected light.
So, in RC, it is separated from the emulsion but in other cases it is mixed in or in direct contact and is not known to cause any effect, opinion to the contrary.
Baryta and Titanox are too yellow for use in both RC and FB support. There is some problem putting PE over the Baryta, but I'm not sure of the details. I know that work with Baryta in RC was abandoned. Both are used with appropriate tints to correct for the colorations in their respective supports. In fact, warm tone papers are often achieved by the appropriate tint.
I'm Getting ready to prepare the the formula from http://www.thelightfarm.com/Map/DryP...ateSection.htm and was wondering if anyone could awnser a few simple questions.
First When I add solution B to solution A (the AgNO3 to the Gelatin and such) Is the solution B still in the water bath?
Second How do I set and shred the Emulsion?
And Third Do all three washes need to use distilled water?
and help would be greatly appreciated.
1) Yes. And stir Solution A (the gelatin solution in the water bath) continuously during the entire addition step of Solution B - which should take about 10 minutes to add.
2a) To set it, put the beaker of emulsion in a water bath at 50F or colder - water with ice in it will work. You can use tap water here. You can keep changing the bath as it warms up. You don't need to stir for this step, just let the soution sit in the bath. When the emulsion gets cold enough, it will solidify like jello. Make sure it sits long enough to set all the way through. You can also put it in a light-tight container and store it in the fridge overnight.
2b) To shred it, get a "ricer" (a device used for mashing potatoes). I find it's a little easier to rice the emulsion if it is not refrigerator cold - let it warm up on the darkroom counter top for 15 minutes or so and then rice it. I think the lightfarm instructions have you rice it into a large container and have the shredded emulsion loose in the container, and use a piece of plastic window screening to strain the shreds when you change the water.
I like to rice it into a nylon stocking (knee highs work great). Stretch the stocking over the mouth of a beaker or container filled with chilled or ice water so that the riced emulsion falls into the cold water. When you get all the emulsion into the stocking, remove it from the beaker, and then tie the end shut and then wash it. it will take a few more water washes if the emulsion is tied in a stocking as you need time to let the water diffuse into the emulsion.
3) You can use tap for this step, but then use distilled for the last. If you like, you can use distilled for all.
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The best thing to do is just to make an emulsion, test it and go from there. There wil be plenty of time to worry about trying diferent techniques,formulas and additives as you go on.
The way I shread and wash my emulsion is to put it on cheese cloth and wrapit up with one layer. Bring the ends together and twist them a little for something to hang on to. I then use the same stainles steel film tank with some tap water (about 2/3 full) to squeeze the emulsion into. Be sure your hands are CLEAN so as not to contaminate the emulsion. I place the lid on the tank and adjitate it every 30 seconds or so, after about 2 1/2 minuets (longer wont hurt) I go back to safelight conditions and remove the lid, place a piece of plastic window screen over the opening, hold on to it tightly around the tank and pour out the water. Refill with water,replace lid,etc,etc. The emulsion bits shuld be rinsed off the screen before the next use so there will be no exposed pieces of emulsion getting in with the rest.
I did get a ricer and it workes very well except that it is just a little over size and will not fit into the film tank.
P.S. Hope this helps.
Comments in response to questions about baryta, Titanox (a trade name for TiO2), and RC (polyethylene (PE)) -- Regarding negative impacts of TiO2 on RC cracking, briefly, PE is subject to oxidation which can reduce the chain length, increase the brittleness, and lead to cracking. TiO2 can magnify this effect by providing a source of free oxygen on exposure to light. This has been controlled by stabilization of the PE through the use of antioxidants and careful selection of TiO2 and other additives. The reason for the choice of TiO2 as the internal pigment is due to its high refractive index which provides high image sharpness. Baryta (barium sulfate) has a lower refractive index. Clear (unpigmented) PE gives prints of poor image sharpness. I'm not familiar with problems in putting PE over a baryta layer -- I can only say that there is no apparent need for a baryta layer with TiO2 pigmented PE -- the original intent of the RC layer was to replace the baryta layer. Finally, TiO2 is commonly used as an internal pigment in the raw base of photo products (as is calcium carbonate and to a smaller extent kaolin clay). There are several reasons for this including the contribution to opacity and to whiteness of the back of the print. Regarding the effect of adding pigments to the emulsions, barium sulfate was used as the matting agent for many B&W products, added to the emulsion prior to coating (with no adverse photographic effects).