Making a daguerreotype using mercury development is quite challenging and can be a source of great frustration. Numerous variables are involved and getting consistent results often remains for veterans a carrot on a stick. Robert Shlaer put it best: on any given day, anything might happen.

The sequence is not that complicated. The polished plate is first exposed to iodine vapors, then bromine vapors, and finally back over the iodine. This makes the plate sensitive to light. After the picture is taken, the plate is developed over hot mercury vapors and the plate is cleared of iodine in hypo. With luck, a picture is there. Finally, the plate is gilded which gives the image visual strength and physical durability.

The difficulty is there is not a formula or set of detailed instructions to follow that everyone uses other than the sequence provided above. The dozen or so of us who make daguerreotypes find we all adhere to different details in spite of the fact we are doing exactly the same thing. Some reasons for this are obvious: different size fuming boxes with different amounts of chemicals of varying strength. Add to this temperature, humidity, barometric pressure and general weather conditions, plus other unknown factors each and all creating their unique situation. A few years ago I moved from one house to another, a distance of three miles, and this totally changed all my times and temperature requirements--it was impossible to get a picture with my previous settings. The bottom line is much testing is required and record keeping so that your unique standards can be set.

Examples of fuming boxes

The iodine thickness on the plate controls the contrast of the image and to some extent the sensitivity. Go to the Becquerel section to see examples of iodine colors. Any of these colors work and will make daguerreotypes. The initial colors will give images more contrast with deep blacks and as the layer gets thicker, the contrast lessens. The initial light yellow is the most sensitive and the sensitivity decreases until the onset of the second cycle yellow where it the sensitivity increases. Once the 2nd yellow begins to turn orange, the usefull range for mercury ends.
A couple ounces of iodine crystals in the fuming box at around 70 degrees F should be adequate to reach any of the colors in 15 seconds to 2 minutes. A colder box will take much longer. Also it is a good idea to warm the plate before sensitizing for a more even coating. Typically the first two plates will get to a color faster than the next several and after the fourth or fifth, additional time may be required. Keeping a record of the times and temperatures is helpful.

After the desired color of iodine is achieved the plate is put into the bromine box where things get trickier. The goal is to get the right balance between the iodine and the bromine. Too little or too much bromine and things won’t work. So there is what might be called a window or range of time/concentration of bromine that is needed. The difficulty is the window is very narrow with thin coatings of iodine and widens as the iodine layer gets thicker. It is best to start with a color of iodine that is after the yellows and orange. A starting point for bromine is to look for a color shift that is just enough to just change the color from, say, reddish to reddish/purple. Keep track of the time needed to do that and then on the next plate do test strips of different times near the first time using thin glass, aluminum, or mylar to block the bromine on the plate to get an exact time for bromine. This may have to be repeated numerous times and many times later as the conditions change. An Example

Once an iodine/bromine balance has been achieved, it becomes routine to observe the iodine color and time the bromine. To this point lights, however bright, are not a problem and some believe that some white light helps increase the sensitivity. Set up the routine so you have the lights on the same for each plate. The second iodine needs to be done under a safe light. The amount of time can be from a couple seconds to several minutes. Longer second iodine will lessen bluing in the solarized areas but also decreases the sensitivity somewhat. Some experiments will be needed to understand and control the action on your particular plates.

Camera exposures have two considerations. First, daguerreotypes have very narrow spectral sensitivity--they are sensitive to blue, white, and ultraviolet light. All other colors record very faintly or not at all. Second, the exposure range is about 1.5 f-stops, a very narrow window of opportunity. Many experiments will be required, but start with a white object in direct sun, use a spot meter and keep notes. Measuring a white object, an EV of 16 should record the object in 1.5 to 2 seconds at f 5.6. If exposure times are inconsistent from plate to plate it is quite likely that the polish is inconsistent. Polish can contribute one or more f-stops in plate speed.

The development time over mercury vapors varies considerably with each and every mercury unit. The factors include the size of the unit itself, the volume of air in the mercury chamber and the distance from the pool of mercury and the plate. My standard is 4 minutes at 145 F and another daguerreotypist uses 190 F for 10 minutes. Testing, using timed steps on the same plate, is the only way to get to the correct time and temperature. If specks of mercury show up in the blacks, back off on the time or temperature. A faint image could mean under development or under exposure in the camera. Once a decent image has developed, maintain the time and temperature. All subsequent problems an usually be traced back to polish, bromine balance, or exposure.

Examples of mercury units