PDA

View Full Version : I have a Question--Direct Positive emulsion



gri6o
07-28-2011, 03:16 AM
Hi I'm new here.
I have a Question
Is there a recipe for positive emulsion (something like ILFORD HARMAN) and what if it is no secret

musila
11-12-2011, 12:55 PM
If you don't mind crude imagery and longer exposures, check out the Calotype Society group on Flickr. There's some information on old fashioned direct positive coatings over there.

Steve Smith
11-12-2011, 02:22 PM
I might be wrong, but I would think that if a monochrome film emulsion can be processed as a positive (which it can) then the same should be true of a paper emulsion.


Steve.

Photo Engineer
11-12-2011, 02:24 PM
There are many direct positive emulsion formulas, but they are very complex and require exacting methods of precipitation.

Although I am peripherally aware of them, I am not very familiar with them and have never made one.

PE

jnanian
11-12-2011, 09:09 PM
why not just make tintypes ?
probably a lot less work in the end ..

good luck !
john

holmburgers
11-15-2011, 03:32 PM
To quote PE from another thread...


There are at least two types of direct reversal emulsions. The process is Develop, Stop or Rinse, and Fix using normal chemistry.

The two types in common use are Reversal F and Reversal P which differ considerably. These emulsions are VERY complex and beyond the scope of any discussion so far on APUG, but to make it very simple, A reversal F emulsion is usually say a pure chloride cube which is then treated with bromide and made into an octahedron kind of shape. The cur of this, the chloride, may be fogged, sensitized some way or just left as is before shelling. This process is called conversion and does not make a true core shell emulsion.

That final emulsion then is sensitized as normal but uses a special chemical called a nucleating agent added to the mix.

At exposure, exposed areas will not develop, but unexposed areas will develop thus giving us a positive image directly with a normal developer.

There are two disadvantages. One is that the nucleating agent can build up in the developer changing both direct reversal results with time, and also it can affect negative emulsions so the developer is best used one-shot or only for direct reversal emulsions with lower capacity. The second disadvantage is that these materials have a shorter tone scale and higher fog than normal materials. This results in re-reversal or combined negative and positive images in the same frame. This is rare, but very ungood when it takes place.

Reversal F was used in Kodak PR-10 instant films and in Ektaflex R material.

PE

This is from this thread. (http://www.apug.org/forums/forum37/83235-direct-positive-paper-how-does-work.html)

Jeff Searust
11-15-2011, 03:42 PM
the basic theory of silver emulsion photography is reversal from the beginnings of the process in the early 1800's til today. If there were an easy way to do positives, don't you think everyone would be doing it?...

John Shriver
12-22-2011, 10:47 PM
There are direct positive duplicating microfilms, such as Kodak 2468. I got a few 100' 35mm rolls for almost nothing, the hitch is the stuff is incredibly slow, ISO around 1 or slower...