They didn't - they used ripped pieces of fabric and cloth to wipe themselves. They used the sponges to clear obstructions, somewhat like a combined toilet brush / plunger.
> The researcher Gilbert Wiplinger put forward a theory on the use of the xylospongium and it seems much more credible. He suggests it was used for secondary cleaning of ancient lavatories in a similar form in which modern toilet brooms are used.
> The discovery of scraps of cloth in an ancient septic tank in Herculaneum led also environmental archaeologist Mark Robinson to conclude that scraps were used for wiping instead of a sponge.
Take a look at the Spa Sanitas Per Aquam conference proceedings from 2009 for more details.
I would have assumed that old fabric was a valuable resource. Used fabric is essentially rubbish now so we don't see that. Ragman or rag merchant used to be a job. Only the very wealthy would use rags that way I would guess.
> The discovery of scraps of cloth in an ancient septic tank in Herculaneum led also environmental archaeologist Mark Robinson to conclude that scraps were used for wiping instead of a sponge.
