Fireclay crucibles are made from coarse fireclay powder (100-500 microns) and extremely fine kaolin clay powder (binder). A powder mass for a fireclay crucible consists of approx. 80% fireclay powder and approx. 20% kaolin binder. In addition, the mass contains 2-5% water if crucibles are pressed and up to approx. 20% water if crucibles are plastically formed.

Fire assay crucibles are generally made from materials that withstand high temperatures and repeated heating and cooling cycles. The most common materials used include clay, graphite, and silica, with some additives to enhance certain properties like thermal shock resistance, corrosion resistance, and non-wetting properties.

Regarding particle size granulation, a good starting point is combining different particle sizes to get a dense and strong structure. Coarser particles (within the range of ~1 mm) can increase the overall strength of the crucible. In contrast, finer particles (on the order of micrometres) can fill in the gaps between the coarser particles and enhance the density of the crucible, thus improving its thermal shock resistance.

However, it's essential to note that choosing the correct particle size granulation is only one aspect of creating a high-quality fire assay crucible. Here are some additional tips to improve your crucible's performance:

1. Material Selection: As mentioned earlier, the crucibles are commonly made from a mixture of clay, graphite, and silica. The choice of material and their proportions significantly influence the properties of the crucible. Different materials might require different granulations for optimum performance.

2. Mixing and Forming Process: How you mix and form your crucible can significantly affect its final properties. It's critical to ensure that the materials are mixed thoroughly to achieve a homogeneous distribution of particle sizes. The forming pressure and method (pressing, slip casting, etc.) can also affect the crucible's density and strength.

3. Drying and Firing Process: The crucible should be dried slowly to prevent cracking due to the rapid evaporation of water. Also, the firing process should be carefully controlled: a slow heating rate, holding at certain temperatures (biscuit firing), and a slow cooling rate can all help to prevent cracking.

4. Quality of Raw Materials: Make sure to use high-purity raw materials, as impurities can cause cracking or other defects during firing.

5. Design of the Crucible: The thickness and shape of the crucible can also affect its thermal shock resistance. A thicker crucible can generally withstand thermal shock better than a thin one but also requires a longer heating time.

In conclusion, improving the performance of a fire assay crucible involves careful consideration of many factors, and it might require some trial and error. If problems persist, consider contacting ceramics manufacturing or materials science experts for more detailed advice.

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