I believe that a few Duralogs burned into ashes and then used on your edible plants is perfectly safe. I would  recommend that you add some fallen leaves, kitchen waste, coffee grounds and other garden waste to the ashes when you add them to your soil. When you add organic matter you are adding tiny critters that help break down waste into a form that plants can use as nutrition. The more varied the material you use the better for your plants. Tomatoes love ashes and other forms of organic matter. In traditional farming practices ashes from fireplaces are scattered on top of the soil in the fall to break down all winter and improve the soil for spring planting. Keep growing, Marianne Binetti