Well, to connect this all to the Polaroid thread we started from, here is a simplified view of how Polaroid B&W works.

One sheet is a negative film the other is a sheet of paper with baryta or titanox overcoated with a layer of neutral colored colloidal silver that is so finely divided and so low in coating weight that the dmin is not turned gray or discolored.

You take a picture and spread a goo between these two sheets. The goo must be bubble free and is composed of a fast acting monobath. The goo must be spread evenly by means of rails and rollers to get an even coating.

As the negative image develops, the fixing agent dissolves the positive image which is silver halide and it diffuses to the reciever sheet with the colloidal silver. There, the colloidal silver acts like a uniform latent image waiting to happen and the dissolved silver halide begins to form a positive silver image. BINGO! Positive print!

Now, the image that forms is colloidal, has a silver halide solvent present, and is therefore subjet to bronzing and fading so you have to coat it with something to "tone" it. This is the little applicator that Polaroid supplies.

Now, where does the speed come from? Assuming a 50 ISO negative, if it were coated on paper, it would be ISO 100 due to back reflection and due to the chemistry involved, you mainly get the toe silver from the negative which is about ISO 100, you can get a print of about ISO 200 from an ISO 50 negative emulsion.

That is a short description of the process.

Colored colloidal silver can give colored (toned) images to start with. Non-uniform colloids can give uneven or blotchy images. So, the silver must be neutral and very low in coating weight per unit area and should be the finest of all possible colloids.

PE