There are many references available on the internet regarding consumption of ash from untreated wood. The practice apparently dates into prehistory.
However, as a few people have pointed out, the intent was not to use the ash as a food but as a food modifier.
My own research shows four main uses:
as a leavening agent, both in Europe and in North America prior to the European invasion.
as a detoxifier. Some Native Americans used ash to remove tannins from acorns. The potassium binds with the tannins, resulting in fewer washings.
As a pickling adjunct in Europe. The lye formed by mixing ash and water helps break down the organic tissue, promoting better absorption of pickling agents. The same mechanism was adopted to increase the efficiency of the hide-tanning process.
To increase the bio-availability of B-vitamins in Native American mais.
In conclusion: yes, humans have been using wood ashes as a food additive for thousands of years throughout the world but, no, they have never eaten the ash as a food in itself.
Finally, an invitation to reason. As Karen points out, ash is the product of burning wood at temperatures high enough to break most organic chemical bonds. Scientists have analysed all kinds of wood ashes - the stuff used to have significant commercial value, after all. There is nothing in wood ash that can serve as a source of human calories. We may not like that conclusion, but our likes and dislikes do not alter the facts. Thanks.