Spec-Caps are shallow aluminum cups in which a sample is pressed. A sample disc properly prepared with a Spec-Cap will be encased by the Spec-Cap on one side and around its edge, allowing the disc to be written on and handled without crumbling or contamination. The Spec-Cap also protects the bore of the die from abrasive samples, which with time can enlarge or damage the die, and the paint on the 3615, 3617, and 3619 Spec-Caps acts as a lubricant, making it easier to remove the sample disc from the die.

There are two types of Spec-Caps, unpainted with flared walls (3619A, makes 31 mm discs) and painted with straight walls (3619, 30 mm, makes 31 mm discs; 3615, 33 mm, makes 35 mm discs; and 3617, 38 mm, makes 40 mm discs). The painted, straight-walled Spec- Caps can be used in two ways. The simpler is to fill the Spec-Cap with sample, level it off, and assemble the die around the filled Spec-Cap. This approach allows many samples to be set up and marked in advance, and when successful is quite efficient, but there are handicaps; perhaps the greatest is that unless the sample material is unusually dense or incompressible, the sample disc will be very thin, prone to cracking and possibly (in the case of low-Z elements) less than “infinitely thick.” There is also a chance of the Spec-Cap wall crumpling inward, and the technique of assembling the die around a loaded Spec-Cap requires some finesse.

Alternatively, the flared Spec-Cap technique may be used with either the painted Spec-Caps or the 3619A Pre-Flared Spec-Caps. Here the Spec-Cap is flared before being placed in the die; 3619 and 3617 Spec-Caps are flared by the user with the 3618 Edge-Flaring Tool, 3615 Spec-Caps are flared by the user with the 3625 Edge- Flaring Tool, and the 3619A Spec-Cap is flared at the factory. With this technique the die bore and base are assembled, the lower polished pellet inserted, and the flared Spec-Cap pushed down against that lower pellet. Then the sample is poured into the die, with the upper polished pellet and the plunger following. During pressing, the sample powder is forced inside the Spec-Cap. When the proper amount of sample is added, the top of the sample disc and the edge of the Spec-Cap will coincide. Obviously the sample weight will vary with the density of the sample and the size of the disc, but the general range of sample weight is, for a 31 mm disc, 5 to 8 grams; for a 35 mm sample disc, 7 to 10 grams; and, for a 40 mm disc, 8 to 12 grams.