The acetype process is perhaps the simplest and easiest to carry out, and results in arguably the most beautiful positive images that you will ever see on black and white film.
Tintypes or ferrotypes as they are sometimes known, are metal plates that are covered with a black lacquer that provides the base for a light sensitive silver emulsion. The black background means that once the image is developed, instead of appearing as a negative, the silver in the image actually reflects more light than the unexposed film which reveals the black areas beneath. It is the contrast between the silver and the unexposed areas that determines whether we see it as a positive or negative. You can test this by placing a black and white slide on a black background, or, as in the case of the acetype process, simply paint the back of your film with black paint once it has fully dried.
On an acetype the shadows are glossy due to the reflective film base, and the highlights are of a more matt appearance.
There are only a few points to consider when shooting film for acetype purposes, and the first is film speed. In order to get well exposed acetypes you have to underexpose by at least two stops, perhaps up to four or more. This is because a much thinner negative is needed in order to provide maximum contrast with the black backing, so if your film is 100 iso, you actually have to rate it between 400 and 1600! Therefore, you have to make sure that your camera is capable of shooting at high enough speeds and small enough apertures in order not to overexpose when shooting in sunny conditions.
You will be surprised at how much silver actually gets developed in an underexposed negative, because usually it is not able to make any difference when printing in the darkroom, but becomes very obvious when viewed by reflection as it sits on the surface of an acetype.
The second thing to watch out for is to make sure that you are coating the film side and not the emulsion side. I made this mistake many times and almost gave up on the process due to poor results stemming from viewing the emulsion through the film base, and not on top of it. My problem was due to me wanting to have the image appear the correct way around, and not back to front, so bear that in mind if shooting scenes with obvious text in them.
Acetypes remove the need for the extra equipment needed to make, store and coat photographic emulsion, and do away with the task of coating and then storing plates while you wait for them to dry, which must be carried out in the darkroom, and enable you to use standard film holders instead of plate holders that generally only take one plate at a time, and are often camera specific.
The huge speed increase means that portraits can be made indoors with natural lighting and more than adequate depth of field, along with fast shutter speeds. The sensitivity of modern films easily outdoes both old and current liquid emulsions, meaning that the acetype process can open up a whole new world of possibilities for shooting, that were never before possible. Being able to shoot 8×10 film at f64 and 1/500 of a second is lightyears away from how we are used to handling large format.
Another bonus of the technique is that it can be used to recover badly underexposed negatives that are unsuitable for either printing or scanning. While you may prefer large format negatives there is a certain charm to 35mm and medium format acetypes.