I frequently find golf ball-sized rocks in bags of lump charcoal. Usually after the fire has burnt out.
Is this some by-product of the charcoal manufacturing process or an under-handed attempt by the manufacturer to cheat on the weight of the bags?
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I've often seen small stones in charcoal, and it makes sense.
Charcoal is often a byproduct of logging, made from lower quality wood such as branches and thin growth at the top of trees. During felling and trimming, this will lie in the dirt while the more valuable wood is dealt with. Then it will be picked up by machinery, loaded, and taken away to be burnt into charcoal. Some contamination from soil is inevitable, and that will include a few stones as well as smaller stuff that adds to the dust in the bottom of the bag. Even treestumps can be used, and digging those out is also likely to bring some soil with them.
When buying lump charcoal (which is my preference) I've found that the brand is really important. Some of these producers are sourcing their wood from worksites or other potentially contaminated locations. Keep an eye out for dimensional lumber, for example. You probably don't want charcoal made from pressure treated lumber. But it can get far worse: PVC, metals, and other debris can find it's way into the bag.
In my preferred brand, I have found an occasional rock. Generally, they are roughly the size and shape of a piece of charcoal. While I'm annoyed to pay charcoal prices for a rock that I don't need or want, the quality of the rest of the charcoal is very good. This leads me to think that this is not intentional but just part of ramping up production. I don't think people are looking at every log or stick that's going into the 'cook'. They are probably scoping up wood with a front loader or some sort of large claw and some rocks are picked up inadvertently.
I would keep an eye out for things like plastic but a few rocks are part of the cost of using lump charcoal, IMO. If you are getting a lot of contaminants in your charcoal, look for another brand. Where I live, the smaller hardware stores and butcher shops have the good stuff.