Bell Products: Cast Bronze Bells

The first step in bell making is the mold. Brick is laid up to create a core or inside of a bell. Molding sand is applied over the brick to finish the core.

The finished core of the mold is dried with gas heaters. The core is dried and covered with a white 'parting compound' so that the clay pattern bell, or "false" bell, may be built up upon it.

The false bell is then finished to the strickle-board and covered with a wax coating, which will eventually assure a smooth casting and allow the false bell to be removed easily.

Inscriptions, which have been made from molding wax, are now applied. A finished false bell has the same appearance as the bronze bell will have after casting.

A fine slurry of molding material is made and used to form the first coat of the outer mold. This material must be fine enough to take all the detail from the wax lettering and also to give a smooth finish. It must be applied carefully, a little at a time, with drying in between. The outer mold must reflect accurately the intricacies of the design.

An iron case is then assembled around the built-up bell mold and as each section is added, it is stamped full of molding sand. The material that was previously placed around the false bell and the iron case together form the outer part of the bell mold and are called the cope.

The entire mold is then heated, softening the wax between the false bell and the cope. The cope is lifted off and is set up so that the remaining traces of wax may be removed. It is then dried from the inside. Any remaining traces of wax or moisture could, at the very least, cause bubbles in the final casting, and could cause an explosion when hit by the molten bronze. The iron casting is necessary to withstand the tremendous outward pressure of the molten bronze during casting.

Having done its work, the false clay bell is now broken away from the core. In casting, the space it has occupied in the molding process will be filled with the molten bronze.

When the furnace is tapped, the molten bronze, at a temperature of 2000°F begins to flow through the channels and into the mold beneath. The Master Founder and his assistants control the flow and stand alert to remove any slag or foreign material before they reach the mold. With the casting completed, the Master Founder may take off his gloves. The gasses continue to burn for hours. The cooling of the newly cast bell is slowed and made even by being buried in the earth. It will be left in the ground three weeks in order to assure complete cooling. It will then be dug up and the mold broken away. Only then will the success or failure of the casting be known. This emphasizes the fact that the mold must be built for every bell cast from a 33 ton giant to the smallest carillon bell of perhaps 6 lbs. weight.

The mold is then closed down. The halves must be fastened together very securely, so that the molten bronze cannot escape where they are joined. This mold has now been buried in the earth in front of the great, wood-fired furnace. Channels lined with refractory material have been made to carry the molten bronze directly from the tap to the mold-head.

In casting smaller bells, the bronze may be melted in a tilting crucible gas furnace. This also facilitates the increase in tin content, which is used in smaller bells of carillons, in order to achieve a clear, brilliant tone quality. The tin content is raised to nearly 25% in the smallest bells.

Prior to tuning, the bell is brought to a high smooth bright finish by means of cleaning and burnishing bronze bells.

All bronze bells by Verdin are tuned to high standards. Larger bells must be tuned on a vertical boring mill. Bells are cast slightly thicker than required, that is, with all the partial tones higher in pitch than their correct positions in the scale. By a careful process of removing just the right amount of metal at different levels inside the bell, the partials are then lowered to the correct positions. (Diagram)

A tuning strategy must be laid out for each bell as it comes to the tuning room from the foundry whereby it will be possible to bring all partials to the correct pitches at the same time. Very few partials can be raised in pitch at all, and those only very slightly.

Small bells, such as the upper bells of carillons, and the bells of chronochimes may be turned on an engine lathe. All bells must go through the same exacting process, which is now controlled with the greatest accuracy by means of electronic pitch measuring instruments, reading to 1/100th of a semitone.

This is the process of creating a uniquely finished bell.