Rather than washing their own clothes, people would take them to a cloth-launderer. On rural farms, slaves would do this.
Firstly, the clothes would be placed into small tubs located within niches surrounded by walls. The tubs would be filled with water mixed with alkaline chemicals, most commonly human or animal urine. The urine would be gathered from vessels kept at the corners of the street, filled by passers-by.
The fuller would trample the cloth with his feet, then scrubbing it and wringing it out. The chemicals would remove grease and fats. 'Fuller's earth' would also be rubbed onto the cloth for the same purpose. These 'fulling stalls' are used to identify such 'fullonicae' in archaeological sites, due to their universal use.
The clothes would then be rinsed and washed to remove the chemicals used beforehand. This would be done in large basins connected to the urban water supply.
After rinsing, the cloth would finally be treated in various ways. How this was done varied in certain places and the whims of customers.
In no particular order:
Cloth would be brushed, often with the thistles of plants or the skin of hedgehogs.
The cloth would sometimes also be treated with sulphur. According to the writings of Pliny, so-called cimolian earth was rubbed into the cloth in order to whiten it, having previously been blasted with sulphur steam, in order to destroy colours and maintain the white colour.
The cloth would also be pressed in screw presses, found in the ruins of Pompeii and Herculaneum.
The work of 'fullones' was an issue treated seriously in Ancient Roman society. So seriously, that in 220 B.C, the Censors C.Flaminius and L.Aemilius proscribed into law the manner in which clothing was to be properly washed.
Roman law emphasized the use of 'Cimolian Earth' in order to brighten colours faded due to use of sulphur.
Fullones were legally responsible for the clothes in their possession, and were thus subject to penalties if said clothes were damaged or returned to the wrong person.
Although the work of the Fullo was highly respected, clothes were considered devalued when they were cleaned, the Emperor Elagabalus going so far as to say that he would not touch washed linen. Thusly, it was considered in poor taste to gift to someone an already-washed article of clothing.
The work of the fullones was connected to the Goddess Minerva, and on March 19th, the feast of Quinquatrus was often held within the workshops of the fullones in her name.
Though some evidence exists, modern historical studies doubt the idea that fullones ever treated cloth straight from the loom.