What was the purpose of publishing the open letter to Thomas Auld in 1848?

makes his purpose in publishing the open
letter to Thomas Auld clear in the final paragraph. Auld was his former slave master, from whom
he had run away exactly ten years before he wrote the letter on 3 September 1848. Having written
nine long paragraphs of narrative and , Douglass concludes by writing:

I intend to make use of you as a weapon with which to assail the
system of slaveryas a means of concentrating public attention on the system, and deepening their
horror of trafficking in the souls and bodies of men. I shall make use of you as a means of
exposing the character of the American church and clergyand as a means of bringing this guilty
nation with yourself to repentance.

Douglass says that he
entertains no malice towards Auld personally ("There is no roof under which you would be
more safe than mine") and that he would regard it as a privilege to show Auld how men ought
to treat each other. This was generous of him, since Auld was a cruel master. In fact, the two
did meet almost thirty years later in 1877, after the abolition of slavery, when Auld was on his
deathbed and told Douglass that in his place, he would also have run away.

Besides his public purpose, Douglass had a further personal reason for writing the
letter, which he discloses in paragraph eight:

At this
moment, you are probably the guilty holder of at least three of my own dear sisters, and my only
brother in bondage. These you regard as your property. They are recorded on your ledger, or
perhaps have been sold to human flesh mongers, with a view to filling your own ever-hungry
purse. Sir, I desire to know how and where these dear sisters are. Have you sold them? or are
they still in your possession? What has become of them? are they living or dead? And my dear old
grandmother, whom you turned out like an old horse, to die in the woodsis she still alive? Write
and let me know all about them.

He points out that if his
grandmother is still alive, she can be no possible use to anyone as a slave since she must be
almost eighty and asks Auld to send her to him, so that he can care for her in her old
age.