Lexitechnia

Cato the Elder, De Agri Cultura:

Stercus unde facias: stramenta, lupinum, paleas, fabalia, acus, frondem iligneam, querneam.

You can make manure from straw, lupin, chaff, beanstalks, husks, and boughs of oak and holm-oak.

The short glosses of palea and acus both tend to be ‘chaff.’ Here, however, they are distinguished. Pliny describes the difference:

Acus vocatur, cum per se pisitur spica tantum, aurificum ad usus, si vero in area teritur cum stipula, palea, in maiore terrarum parte ad pabula iumentorum.

It’s called acus when just the spike is crushed by itself, for the use of goldsmiths; and palea if it’s ground on the threshing-floor along with the straw, as used for feeding beasts of burden in most of the world.

Acus is the rarer term and appears to be specifically for chaff from the husks of grain alone, while palea is the general term and includes straw chaff.

The mention of goldsmiths refers to the Roman practice, related by Pliny and Macrobius, of considering chaff (palea) the best material—or, indeed, the only appropriate material—to start a fire hot enough to melt gold.

The Romans weren’t the only people to use chaff in goldsmithing: the Egyptians, likewise, were said by Agatharchides to use this sort of chaff in refining gold. They heated the gold with salt, barley husks, and lead and/or tin to purify it.^[1]