The smelting day typically begins on a Saturday morning and continues into the early hours of the following day. A tatara requires constant attention in it's operation, as it is continually fed with charcoal, iron ore, and bellows driven air. The ceramic materials with which it is built will certainly crack, requiring repair. The air system needs to be kept clear. And molten waste, produced by the melting of ore as well as the furnace interior, must periodically be drained away.
The tatara is brought to life gradually. A variety of paradoxical demands are imposed on it's structure, and by the extreme nature of it's function, it has qualities of both ruggedness and fragility. It is a complex mechanism; composed of ceramic materials that must not only melt, but at the same time contain the molten mixture formed within it's core.
As was done during the drying stage of it's construction, the tatara is warmed slowly. A small wood fire is lit inside and over the course of several hours stoked up to a fierce blaze. Peep holes and tap holes have been left open and thus allow a natural draft to occur. Fuel is fed into the furnace from it's open top.
The chemical reactions causing the reduction of ore to iron occur at temperatures in the range of 2400-2600 degrees fahrenheit. This level of heat is achieved through the combustion of charcoal using a forced draft. At Wapack Tatara the energy of falling water is transferred from rotary to reciprocal motion by use of a water wheel, crank-arm, and connecting rod, and is then used to power a piston-box style bellows.
When the tatara has reached the highest level of heat obtainable through the burning of wood by natural draft, charcoal is introduced; the peep holes are closed off with wooden plugs, and the supply of bellows-driven air begins.
The maximum temperature achievable by the combustion of wood is around 1000 degrees. As consumption of charcoal begins, the forced draft must be increased gradually until the optimum level of heat is obtained: approximately 2500 degrees.
Based on prior experience a burn rate is determined: how much fuel the furnace should consume in a measured period of time. Air volume is the controlling factor - increasing the amount of draft causes the charcoal to burn faster, and hence more heat is released in the combustion process, thus raising the temperature.
The tatara is filled to it's top. As the charcoal is consumed, it drops to a lower position, and more is added.
When the desired burn-rate is achieved, iron ore is introduced. The charcoal-to-ore ratio is one of the variables that will affect the quantity and quality of iron produced by the smelt and is determined through experience gained by trial and error. Charcoal and ore are measured by weight into a charge. Since the tatara was created for smelting fine grain ores such as magnetic black sand, care must be used to prevent the ore from being blown out of the stack by the upward draft. As the materials are consumed, and their level falls, the ore ore is sprinkled evenly over the top surface and then a layer of charcoal is placed over it, filling the tatara back up to its top.
The ore is introduced gradually. Initial charging begins with a low ore/charcoal ratio. And then, over the course of several charges, the proportion of ore is increased until the tatara is running at full charge. As each charge is consumed, the time is noted to assure that the desired burn rate is maintained. Carefully kept records are also an important part of analyzing the performance of each smelt and will help determine future adjustments in variables such as furnace design and operating procedure.
The tatara is a bloomery furnace. It's product is a sponge-like mass of iron that forms at temperatures well below that of iron's melting point. The molten mixture of furnace wall, non-iron material contained in the ore, and unreduced iron oxide is called slag. This substance plays a critical role in the chemistry of iron reduction. Because it is continually being produced, quantities of it must be drained off (tapped) during the smelting process. In order to access the molten pool of slag forming within the furnace core, tap holes must be maintained in such a manner as to allow the creation of a frozen slag dam. However, these need to be kept heat-softened, so that the dam can be readily cut through whenever it is necessary to tap the slag.