RICHMOND, N.H. - Dan Greenspan went from jazzman to breadman, a circuitous path that involved reading about an ancient loaf and striving to perfect it, apprenticing himself briefly to a professional baker in New Hampshire, and having a well-known oven maker build him a customized wood-fired oven on the back wall of what had been a screened-in porch at his house here. Since last September, Greenspan has been producing Dan’s Brick Oven Bread, an old Flemish loaf called desem, a naturally leavened whole-wheat sourdough bread with flour, water, and salt the only ingredients. His weekly yield is 150 loaves, which gives new meaning to both the words artisan and handmade. Greenspan delivers his hearty loaves to the Boston area, where they sell for $5.49 to $6.99.