That article is interesting, but I wish it covered more than the UK and the US and a particular set of working conditions. In the US for example, slaves were forced to work from dawn until dusk, giving them 14-hour workdays in the Summer. I have no idea what workdays looked like in China, India, or outside of the period of history prior to the industrial revolution, but I would suspect the amount of exploitation varied widely even within the narrow times and places we have historical access to.