Antique kilns direct heating, wood or coal

In ancient times, glass was fused in open kilns. The Tell Amarna kilns, dating back to 1350 – 1334 BC, are an example. In such kilns, the fusing process was carried out in two phases. First, the vitreous mix was fused at medium temperature in clay pots. Once the mass had cooled off, the top and lower layer containing impurities were cut off, later, the remaining glass was pounded and refused at approximately 1100ºC. This technique was gradually improved. During the Roman period kilns were greatly upgraded.

Crucible

During the Middle Ages, crucibles were made up with vaulted circular or oval chambers, which were built with refractory bricks capable of withstanding a temperature of 700ºC, surrounded by different crucibles stoked with wood. It is worth mentioning that, since the XII to the XVIII centuries, these kilns did not undergo any changes since the same system is mentioned in all glass treatises. However, since the XVIII the arrangement of the kilns suffered continuos changes to fulfil diverse needs and most of all to become suitable for the new fuels in use, ranging from wood to coal and even the gasses resulting from the combustion of coal. The kilns using such fuel amount for the transition between those using wood to the new gas kilns.

Crucibles are mainly used to fabricate fine glass of medium size.
They have open crucible on top (common glass) or on the side (crystal) that rest on a circular floor. To make this type of kiln the use of heat-resistant clay is required, crucibles of lean or ungreased clay, which resist glass-fusing action without altering their form with walls approximately four inches thick. The many disruptions in the work brought about by the fracturing of the crucibles due to the thinning of the walls and for other accidental causes were the reason for the building of the so-called continuos crucible, which were a three-piece set connected by openings. This type of kilns marked the turning point between the ancient manufacturing processes and the fabrication of tank furnace currently in use.

Crucibles tend to disappear, leaving their place to those fueled by gas and to the industrial tank furnace already mentioned which allows for continuos work to be carried out. Also, now a day, electric furnaces are in use, mostly for the elaborating of fussed glass, mould cast glass, grisalla and other handcraft varieties.

Tank furnace

This type of furnace was invented by Federico Siemens in the XIX century and is used to fabricate highly consumable products. A crucible heated with gas that has the three required areas: fusing, working and annealing, makes up this kiln. The tank is of rectangular shape, 40 meters long, traversed throughout its length by the fusing glass. It is built with high refractory materials, the dome is almost flat and one of its sides is a vertical wall with a trough, the opposing wall is domed with working ends. Its bottom is divided into three areas by two channels crossing the bottom lengthwise so the vitreous mass may be transferred from area to area.

Siemens wonderful invention brought about, since 1870, a series of improvements, resulting in the elimination of two of the channels and the fact that currently the furnace is made out of a single tank. In such a way, fully automatic process furnaces that considerably increase production and reduce labor have been designed.

Large-scale furnaces

These furnaces are heated directly by liquid fuel or heated indirectly by gasogene gas.

In many installations of this type, retrievers of the calories contained in burnt gasses are used. The fusing phases in such furnaces go from 1200 to 1400ºC.