What is a blacken factory as seen in Charles Dickens?

"Blacking" is what we now call "shoe polish." Charles Dickens was sent to work in a blacking factory when he was about twelve. His job was to apply labels to the bottles and cork them. He worked there while his father was in debtor's prison, along with the rest of the family. He was so thoroughly humiliated by the experience, though it only lasted about 18 months, that it colored the rest of his life and remained a dark secret until his death.