Here's a few contemporary descriptions, which give an idea of the condition of the slaves:
Diodorus of Sicily (90-21 BCE)
the slaves who are engaged in the working of them [the mines] produce for their masters revenues in sums defying belief, but they themselves wear out their bodies both by day and by night in the diggings under the earth, dying in large numbers because of the exceptional hardships they endure. For no respite or pause is granted them in their labours, but compelled beneath blows of the overseers to endure the severity of their plight, they throw away their lives in this wretched manner, although certain of them who can endure it, by virtue of their bodily strength and their persevering souls, suffer such hardships over a long period; indeed death in their eyes is more to be desired than life, because of the magnitude of the hardships they must bear.
Diodorus Siculus, Diodorus of Sicily, Loeb Classical Library, translated by C.H. Oldfather, vol. 3, Cambridge, MA: Harvard University Press, 1952, Book V, 37, 38, 199-201.
Diodorus went on to provide the following description of agricultural slavery in his native Sicily:
The Italians who were engaged in agriculture purchased great numbers of slaves, all of whom they marked with brands, but failed to provide them sufficient food, and by oppressive toil wore them out?. There was a certain Damophilus, a native of Enna, a man of great wealth but arrogant in manner, who, since he had under cultivation a great circuit of land and owned many herds of cattle, emulated not only the luxury affected by the Italian landowners in Sicily, but also their troops of slaves and their inhumanity and severity towards them. He drove about the countryside with expensive horses, four-wheeled carriages, and a bodyguard of slaves, and prided himself, in addition, on his great train of handsome serving-boys and ill-mannered parasites. Both in town and at his villas he took pains to provide a veritable exhibition of embossed silver and costly crimson spreads, and had himself served sumptuous and regally lavish dinners, in which he surpassed even the luxury of the Persians in outlay and extravagance, as indeed he outdid them also in arrogance. His uncouth and boorish nature, in fact, being set in possession of irresponsible power and in control of a vast fortune, first of all engendered satiety, then overweening pride, and, at last, destruction for him and great calamities for his country. Purchasing a large number of slaves, he treated them outrageously, marking with branding irons the bodies of men who in their own countries had been free, but who through capture in war had come to know the fate of a slave. Some of these he put in fetters and thrust into slave pens; others he designated to act as his herdsmen, but neglected to provide them with suitable clothing or food?. Because of his arbitrary and savage humour not a day passed that this same Damophilus did not torment some of his slaves without just cause. His wife Metallis, who delighted no less in these arrogant punishments, treated her maidservants cruelly, as well as any other slaves who fell into her clutches. And because of the despiteful punishments received from them both, the slaves were filled with rage against their masters, and conceiving that they could encounter nothing worse than their present misfortunes, began to form conspiracies to revolt and to murder their masters?
Diodorus Siculus, Diodorus of Sicily, translated by Francis R. Walton, vol. 12, Cambridge: Cambridge University Press, 1967, book XXIV/XXXV.2.2, 77-81.