Rusty gear restoration. Non-standard solution

[hemmjo]

Yes it is certain he added the rocks to take up space. However it is really counter productive to use rocks for that. That gravel he put in there looked like a mix of various types of rock so the odds of getting a bad reaction are very good. Calcium-based rocks like limestone are particularly reactive and is actually used commercially to reduce acidity. In fact the crust on that gear appears to be calcium based so adding rocks containing calcium simply depletes the vinegar more quickly. Some Igneous and metamorphic rocks, such as granite and gneiss release toxic aluminum ions. So he created a toxic mixture by adding the rocks.

It would be better to just use more vinegar, or molasses as mentioned above.

Although, not knowing for sure the chemical make up of the crust on the gear, I would not want to dump the residue on the garden. If it was just rust he was removing, the iron might be a good addition to the garden, but I try to keep the unknown chemicals out of my garden