When John Ames writes of ashes, he seems to mean something bad. He describes a sermon that didn't go as planned as "ashes in my mouth," for example. (1.5.29). Burned, ruined, tongue-tied.
But there's more at work in the image. In his childhood, Ames helped gather and save items as a church burned. His father brought him a biscuit dirty with soot. "There's nothing cleaner than ash," his father tells him (1.7.78). Ames thought the ash tasted of affliction, but he learned then not to avoid the soot life brings; that is, he learned that you have to take the bad with the good. You can live with the ash, even if you can't live on it.
In fact, Ames goes so far as to call the soot-covered bread "communion," a holy meal, a symbol of people's connection with the divine (1.7.85):
His hands and his face were black with ash—like one of the old martyrs—and he knelt there in the rain and brought a piece of biscuit out from inside his shirt, and he did break it, that's true, and gave me half and ate the other half himself. And it truly was the bread of affliction, because everyone was poor then. (1.7.103)
The image of breaking bread, of course, goes back to the Last Supper, when Jesus broke and ate bread with his followers, establishing a ritual continued by the various Christian communities. No, wait: the image goes back even further. The "bread of affliction" was the unleavened bread the Israelites ate in memory of their flight from Egypt.
Ash, then, is for Ames an image of hope. It's not opposed to the image of light; it's complementary to light.