Brian, I've made mulberry bows with all sapwood, a few rings of sap left on, 50/50 and all heartwood. They all worked well(as long as you know the sap wood is sound) with the all heartwood being the thinnest and the all sapwood being the thickest. Mulberry with a few sapwood rings(one if its thick) gives nice contrast between the sap and heartwoods. Steve Parker built a mulberry ELB with sapwood back that had a cool yew look to it.