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The apple prunings, from left to right:  basic colour on silk, basic colour on wool, acid modifier, alkaline modifier, copper mordant, iron modifier. - more on modifiers another day.

A mordant is a metal compound which binds the dye colour to the fibre, usually making it brighter and more light and wash fast.

Alum is the mordant most widely used by craft dyers nowadays as it is non-toxic, though can be an irritant and should be handled with care. 
I  will include a skein of wool mordanted with copper in each dye bath - a copper modifier could also be used after mordanting with alum and dyeing.

On the apple dye, the acid modifier has not made any change, the alkaline one has turned the wool gold, as has the copper mordant (it is brighter than it looks in the photograph) and the iron modified skein is a grey/green.

The colour on the skein treated with the alkaline modifier is a bit patchy - probably because there was not enough liquid in the pan for it to move around freely, so parts of the skein have been subjected to more heat than others.