How does castile relate to To Kill a Mockingbird?

The word castile is mentioned
once during Harper Lee's novel To Kill a Mockingbird. It can be
found in Chapter 12 as part of Scout's narration of the rough bath given to her by
Calpurnia prior to their visit to the First Purchase A.M.E.
Church.

She made me soap all
over twice, drew fresh water in the tub for each rinse; she stuck my head in the basin
and washed it with Octagon soap and
castile.

Octagon soap was
a popular soap in America during the 1930s. It was made by Colgate and was known for its
ability as a stain and grease remover. Not only was it used for bathing, but it was also
used to clean stoves and woodwork. Castile is a specific type of soap, usually made with
olive oil; originating in the 16th century (probably in Belgium or France), it was
probably the first type of hard bar soap, used primarily as a body wash, shampoo and
conditioner.