making and using your own iron mordant/modifier for natural dyeing

A mordant is used in natural dyeing to affix the colour to the fibre.  Basically, mordant sticks to fibre, dye stick to mordant, to create a fancy chemical bond.  a thread about mordant safety.  A modifier changes the colour.  For example, if we are using sorrel to dye yarn, we get a yellow.  If we dip the yarn in an iron solution, it changes the colour to olive green.  Although iron has some moderating qualities, its tendency to drastically change colours makes it more a modifier than a mordant.  

Iron solution is perhaps the easiest mordant to make at home.

Most recipes go like this:

a handful of rusty iron bits
put in a glass jar
fill one-third with white vinegar
fill the rest of the way with water
put lid on (not too tightly as the chemical reaction produces some gas)
leave it a week or more

Sadly, that's where the instructions stop.  I haven't found a book yet that tells me how to use it, so I've been experimenting.  I put a small splash in a gallon of water and then topped up the iron solution with water and vinegar (leaving the nails in it).
It's been surprisngily successful as both mordant and modifier.  But the iron water still has a strong effect on the dyed yarn, so I'm nervous to dispose of the water until the iron is exhausted (used up).  

Anyone else tried this?

A few more things I learned in my reading this morning:

Too long a dip in the iron bath can harm wool.  But I wonder if the heat plus iron is more the problem because most online advice says to limit it to 10 min or less, whereas my books say up to an hour so long as it's not heated to more than steaming.  

When it comes to disposal, the general consensus online seems to be that it's just rusty water, pour it on the garden.  

I have done this for use as a natural wood stain. So you can always use it for that if you dont want to throw it on the garden. I now prefer to use immature walnuts as a wood stain as the tannins make a nice colour for the wood, so maybe that is worth experimenting with for the wool?

Apparently, this solution gets frothy and spills out of the jar in warm weather.  The stuff that spills over stains the table something fierce.  Now I'm leaving the jar only 2/3rds full.  

Update on my iron mordant.  

After some more experiments, one to three drops is enough for about a kilo of wool.  

I also need a new lid as it ate through the mason jar lid.  I'm looking for one of those glass ones with the rubber ring.