The cupel is a shallow, porous dish made of bone-ash, Portland cement, magnesia or other refractory and non-corrosive material. (Bugbee p. 89)

Cupels should not crack when heated in the muffle and should be so strong that they will not break when handled with the tongs. Good cupels give, a slight metallic ring when struck together after air-drying. It is best to heat cupels slowly in the muffle as this lessens the chance of their cracking.

A good cupel should be perfectly smooth on the inside, and of the right porosity. If it is too dense, the time of cupellation is prolonged and the temperature of cupellation has to be higher, thus increasing, the loss of silver. If the cupel is too porous it is said that there is danger of a greater loss, due to the ease with which small particles of alloy can pass into the cupel. The bowl of the cupel should be made to hold a weight of lead equal to the weight of the cupel.

The shape of the cupel seems to influence the loss of precious metals. A flat, shallow one exposes a greater surface to oxidation and allows of faster cupellation; it also gives a greater surface of contact between alloy and cupel, and as far as losses are due to direct absorption of alloy, it will of course increase these.

The writer, using the same bone-ash and cupel machine, and changing only the shape of the cupel, has found shallow cupels to give a much higher loss of silver. In doing this work it was found harder to obtain crystals of litharge with the shallow cupel without freezing, and it was very evident that a higher cupellation temperature was required for the shallow cupel. The reason for this is that in the case of the shallow cupel the molten alloy is more directly exposed to the current of air passing through the muffle, and consequently a higher muffle temperature has to be maintained to prevent freezing.

Magnesia cupels are very hard, which is an advantage in that they do not suffer so much breakage in shipment. They are always factory-made and are decidedly more expensive than bone-ash cupels, which may be home-made. Certain brands of magnesia cupels give an apparently lower loss of silver in cupeling than can be obtained with bone-ash cupels but it is a question how much of this is real and how much due to an increase in the amount of impurities retained in the silver beads.

Magnesia cupels behave quite differently from ordinary bone-ash cupels, and the assayer who is accustomed to bone-ash cupels will have to learn cupeling over again when he starts using those made of magnesite. This difference in behavior is due mainly to the different thermal properties of the two materials. Both the specific heat and the conductivity of magnesite are decidedly greater than those of bone-ash, so that with cupels of both kinds running side by side, the lead on the magnesia cupel is comparatively dull while that on bone-ash is very bright. This is due to the greater conductivity of magnesite, which allows a more rapid dispersion of the heat of oxidation of the lead, with the result that magnesia cupels require a higher muffle temperature than do bone-ash cupels. An especially high finishing temperature is required for magnesite cupels, to insure the elimination of the last 1 or 2% of lead. A bone-ash cupel will finish in a muffle, the temperature of which is sufficient to cause uncovering, but this is not true of the magnesia cupel, because in this case the heat of oxidation of the lead is diffused too rapidly and is not conserved to help out at the finish.

Magnesia cupels absorb about two-thirds of their own weight of litharge, those of cement about three-fourths of their weight of litharge.

火试金灰皿是一种浅而多孔的盘子，由骨灰、波特兰水泥、氧化镁或其他耐火和无腐蚀性材料制成。
在马弗炉中加热时，灰皿托不应破裂，并且应足够坚固，以便用钳子搬运时不会破裂。好的灰皿在风干后敲击在一起会产生轻微的金属环。设计好在马弗炉中慢慢加热灰皿，这样可以减少破裂的机会。
一个好的灰皿内部应该非常光滑，并且具有适当的孔隙率。如果密度过大，则灰吹时间会延长，灰吹温度也会升高，从而增加银的损失。如果灰皿太多孔，据说会有更大损失的危险，因为合金微粒很容易进入灰皿中。灰皿的碗应能容纳与灰皿重量相等的铅重量。
灰皿的形状似乎会影响贵金属的损失。平坦、浅的表面暴露于更大的氧化环境中，并允许更快的灰吹；它还提供了更大的合金和铜灰皿之间的接触面，至于损失是由于合金的直接吸收，它当然会增加这些损失。
使用相同的骨灰和灰皿机器，只改变了灰皿的形状，浅的灰皿会使银损失更大。在进行这项工作时，使用浅灰皿很难在不冷却的情况下获得氧化铅晶体，很明显，浅灰皿需要更高的灰皿化温度。原因是在浅灰皿的情况下，熔融合金更直接地暴露在通过马弗炉的气流中，因此必须保持较高的马弗炉温度以防止冷区。
镁质灰皿非常坚硬，这是一个点，因为它们在运输过程中不会遭受太多破损。它们都是工厂制造的，肯定比骨灰灰皿贵，因为骨灰灰皿可能是自制的。镁质灰皿在镀银过程中的银损失明显低于骨灰灰皿。

镁质灰皿的性能与普通的骨灰灰皿大不相同，习惯于骨灰灰皿的化验师在开始使用菱镁矿制成的灰皿时，必须重新学习使用铜。这种行为上的差异主要是由于两种材料的热性能不同。菱镁矿的比热和电导率都明显大于骨灰，因此，当这两种灰皿并排运行时，菱镁矿灰皿上的铅相对暗淡，而骨灰灰皿上的铅非常明亮。这是由于菱镁矿的导电性更高，从而可以更快地分散铅的氧化热，因此镁灰皿比骨灰灰皿需要更高的马弗温度。菱镁矿灰皿需要特别高的精加热温度，以确保消除设计后1%或2%的铅。
而骨灰灰皿将在马弗炉中完成，马弗炉的温度足以导致露出，但镁质灰皿并非如此，因为在这种情况下，铅的氧化热扩散得太快，无法保存以帮助完成。
氧化镁灰皿吸收约三分之二的自身重量的锂，水泥灰皿吸收约四分之三的重量的锂。