The day of Jehovah's vengeance - her streams will be turned into pitch, and her dust into brimstone, and her land will become burning pitch. Isa 34:8, 9.

'Pitch' and 'brimstone' stand for falsities and evils. This then is why 'she coated it with bitumen and pitch' means good mingled with evils and falsities.

[2] As regards the actual matter under consideration here, which is that encircling God's truth there was good mingled with evils and falsities, no one can have any understanding unless he knows what goes on during the reformation of a person. While he is being reformed he is, so far as his internal is concerned, preserved by the Lord in a state of goodness and truth; but so far as his external is concerned he is let into his evils and falsities. He is as a consequence among spirits from hell who are governed by such evils and falsities. These spirits flit around him, trying in every way to destroy him. But the goodness and truth that flow into him through his internal give him such protection that the spirits from hell cannot harm him in the slightest, the reason for this being that what acts at a more internal level is immensely superior in strength to that which acts at a more external level. For being purer what is more internal acts on every individual detail of what is more external, and in so doing it adjusts the external to answer its beck and call. But there must in that case be goodness and truth in the external in which influx from the internal can be firmly established. Good can then be among evils and falsities and yet remain safe from harm. Everyone who is being reformed is brought into this state, so that the evil desires and false ideas that govern him are removed and good desires and true ideas are implanted instead.

[3] Without knowledge of this arcanum no one can begin to know why Divine Truth residing with a person has around it forms of good mingled with evils and falsities, meant by the bitumen and pitch with which the little ark in which the infant had been put was coated. It should in addition be recognized that good can be mingled with evils and falsities, but they are not on that account joined together, since one shies away from the other and in keeping with a law of order they separate themselves from one another. For good belongs to heaven and evil and falsity to hell, and therefore just as heaven and hell have been separated, so also does every single thing from them separate itself.