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        Fusion in a muffle furnace

          Because of good temperature controllability and its usefulness, the most popular fusion equipment is the muffle furnace. Disadvantages are a long heating-up time and high energy consumption in continuous service. Fibre-insulated laboratory furnaces with heating offer certain advantages but reference must be made to the problems of fibre materials in fumes from fusion agents.

          Graphite, glazed graphite and Pt crucible are most often used.

          Graphite crucibles have thick walls; fusion times are about 5 min. Loose coal powder can lead to uncontrollable reduction processes.

          Glazed graphite crucibles have thin walls; fusion times are about 2 min. Their service life is about 30 melts which, at a unit cost of 30 DM, is equivalent to 1DM/melt.

          Pt crucible with 5% Au reduce the wetting ability of the melt; the crucibles can be completely emptied. Fusion time is 1 min, which is worth noting for high-speed analyses. The cost of a melt is about 1 DM since, depending on types of sample and degree of dilution, it is possible to make about 100 melts and repair costs are about 100 DM. The purchase cost is high, being about 1000 DM per crucible.

          The following working procedure is recommended:

          Grind the sample, and mix part of it with the buffer in a ceramic dish, using a glass rod. Melt the mixture in the muffle furnace at 1200 ℃. Empty the melt into the container of the swing mill and grind for about 10 s. It is important that the crucible should be completely emptied. Fusion times become shorter with increasing dilution.

          Sr is added as a reference, in the form of SrO. Other elements can also be used, but they should not be contained in the raw sample or be introduced by the bath movement in the form of granules.

          In processes using powder no further sample preparation is required. The powder can be charged into cup electrodes or measured by tape machine.

          In briquetting processes further steps are necessary, as discussed in the case of sample preparation without fusion.

          If the fusions are made in the graphite crucible, they can go with the melt into the swing mill. This is a simple and rapid working method. The grinding tools must have smooth surfaces so that no ground material is left behind to become mixed with the next sample material.

          There are machines equipped with a robot which carry out these steps automatically, including crucible cleaning.


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