Bone ash is a valuable fertilizer, among other uses. In human cremation it is desired to break down even the densest bone; if one simply wants to dispose of old animal bones and convert them into something more useful, it’s not so big a deal if a few recognizable chunks remain afterwards, and the fire needn’t be so intense.
Here are some additional citations that make it clear we’re talking about burning bones:
1493 Festyvall (W. de W. 1515) 105 In worshyppe of saynte Johan the people waked at home, & made iij maner of fyres. One was clene bones and noo woode, and that is called a bone fyre. a1552 in Leland Brit. Coll. I. p. lxxvi, In some parts of Lincolnshire..on some peculiar nights, they make great fires in the public streets of their Towns with bones of oxon, sheep, &c. which are heaped together before. I am apt to believe..that from hence came the original of Bonefires. 1586 MARLOWE 1st. Pt. Tamburl. III. iii, Making bonfires for my overthrow. But, ere I die, those foul idolaters Shall make me bonfires with their filthy bones. 1684 DINELEY Dk. Beaufort’s Progr. Wales 154 A fire of joy..called a Bonfire..being part wood and part bones.