The making of an enamel sphere begins with a thin metal disk whose rim has been turned up. The metal varies depending on the planned value of the finished product. The least expensive version is made of copper. The small temporary feet are first soldered onto the disk. The puck is then flattened to make sure it is straight and the feet are aligned.
Impurities are removed in an acid bath (nitric, hydrochloric, sulfuric). Larger pieces can be fired or annealed to remove any protective layer of oil before acid bathing. A dirty base plate can crack the enamel or make it porous. Pickled motherboard must be thoroughly rinsed. Acid contamination can cause transparent colors to become cloudy.
Cadrans Donze SA receives the enamel powder in large barrels from central Europe. Mr. Vermot-Donze sifts the powder and then washes it in a large glass container. As you may recall, polish is glass and using a metal or plastic container or stir stick will contaminate the polish with pieces of the container or stirrer. The enamel is washed repeatedly, up to 30 times, until the water remains clear and the enamel on the bowl shines. Without washing it, the baked enamel could look grainy, cloudy, or dirty. Since prolonged exposure to water will cause the base polish to deteriorate faster than normal atmospheric conditions, it is recommended that you wash only the amount needed for that day.
The next step in the process is to spread an even amount of less pure glaze over the bottom of the disc. Enamel powder is dusted through a sieve and then placed on an asbestos sheet. The sheet is placed in a preheated oven and left for a few minutes until the powder has melted and forms a shiny coating. Counter-glazing, or back-glazing, is necessary to prevent the finished dial from cracking or the glaze from chipping off later.
The temperature inside the oven is about 850° C (1530° F). Experienced enamelers preheat the kiln 18° to 90°F (10° to 50°C) higher than necessary, due to heat loss when the kiln door is opened. Oven temperatures can be monitored with a pyrometer, but they can sometimes get stuck or require recalibration. Thus, direct observation of the firing and color of the oven is the best monitor for the craftsman. With experience, it never fails.
The final evaluation of the firing time rests with the enameler’s mind, as it depends on factors such as the size or insulation of the kiln, the internal and external temperatures of the kiln, the size of the piece glazed, as well as the glaze and speed. workload of the enameler when placing items in the kiln.
The upper (front) side of the black dial is dusted with a very fine grade enamel powder. This is repeated several times and the black is fired after each layer. The number of layers depends on the required thickness of the finished dial. The usual minimum is three layers. Too thick a coat can cause the polish to crack or flake. Too thin a layer will burn.
The clock mark and numbers are printed using a dial printing machine. Its principle is the same as in any dial factory. The image (figures, logo, name, etc.) is engraved or engraved on a steel plate. Black enamel mixed with a binding agent (usually an oil) is applied to the etched areas and removed with a soft gelatin pad. When the cushion is pressed against the black dial, it leaves the image on it. The image must be fired to be permanent, but the temperature will be approximately 180°F (100°C) less because a higher temperature would cause black enamel watches to cool brown.
After several firings, the enamelled piece can deform. This can be corrected by rubbing the sphere with a flat carbon brick when the sphere comes out of the kiln. This stage requires experience, because if the sphere is still too hot, rubbing will damage the still soft surface. If the dial gets too cold (below 540°C/1000°F), the enamel may crack when pressed.
The sphere is then placed on a template to drill the center hole. The hole is made with a fast turning diamond drill bit, using water as a lubricant. If it is necessary to enlarge the hole, it can be done with a fine conical stone and a file. Each pass of the file should be made from the front to the back of the quadrant. Otherwise the enamel on the front will crack and the dial will have to be rejected.
The sphere is then gently placed in a machine so that it is held by its central hole. The machine rotates the dial while the abrasive disc moves in the opposite direction around the edge of the dial. Grinding is done at high speed, again using water as a lubricant. The grinding wheel centers the outside according to the center hole and cuts through the folded edge that was holding the polish in place in the first place. At the same time reduce the sphere to the required diameter. Edge beveling can be done at this stage.
If the dial is equipped with subdials, the holes for them are drilled below. The holes are the exact size of the subdials. The subdials are made in the same way as the main dial. The only difference is the thickness that allows a subdial to sink slightly below the surface of the main dial. Mr. Vermot-Donze often makes a series of subdials in different styles for future use. For moon phase dials, a subdial can be a combination of a partial subdial and a shaped hole.
The outer edges of the drilled subdial holes are chamfered with a stone from the front. The backs of the main dial and each sub dial are then chamfered to expose the metal plate in the middle of each component. This is necessary because the parts need to be soldered and the solder does not stick to the enamel. Since the subdials are chamfered in the opposite direction to the main dial, the pieces form a V-shaped groove that is filled with soft solder.
Vermot-Donze spent six long years experimenting with different techniques before mastering this method. When he inherited the business from his mother-in-law (Francis Donze), the art of enameling had died and there was no one to explain how certain things were done.