I have never had highly predictable results with the vinegar stain. the color you get is often a reaction between the wood and the stain. Put the same stain on maple from different trees is going to yield different results.
This stain really needs to sit on the shelf and cook for a long time. Get yourself a batch going, and then let six months go by. Age seems to improve it. Add vinegar when needed.
Notes from John Cholin, from the old ALR:
For those of you curious about cider vinegar/iron nails stain I have posted on the topic a couple of times. I don't know if those posts were saved or tossed.
About 20-some years ago I read, some where I wish I kept the article, that the old timers stained stocks with a mixture of cider vinegar and iron nails. I was building my first rifle at that time so I took a jug of apple cider that had gotten "randy" tossed in some nails and let it work. Ever since then that has been my stain.
When you make aqua fortis/iron stain what you are really doing is making iron nitrate with some excess acid. Nitric acid has a very low vapor pressure at room temperature so it does not evaporate from the wood very fast. Once in the stock the iron nitrate gives up the iron ion to oxygen in the air which becomes one of the 3 iron oxides (ferric oxide, FeO3, ferrous oxide, FeO or ferrosoferic oxide fe3O4iron oxide). Heating the stock accellerates these reactions. The nitrate ion bonds with a hydrogen in the wood and has to evaporate or be neutralized and washed away.
You can make the iron oxide with acetic acid too! Acetic acid is a weaker acid than nitric so you have to give it more time to combine with the iron and turn the iron into iron ions that are in solution as iron acetate. The nice thing about acetic acid is that you can drink it or sprinkle it on your salad with out much adeiu. It also has a much higher vapor pressure so it evaporates out of the wood quite quickly. If the stock does not smell like salad the acid is gone and all of the iron ions have become iron oxide. Thus there is no need to neutralize unless you can't wait another hour.
Ferrous oxide, FeO, is black to blackish brown. Ferric oxide, Fe2O3, is orange to red to readish brown. Ferrosferic oxide, Fe3O4, is brown. With the iron acetate (cider vinegar/iron nails) stain you can modify the color by changing the relative concentrations of the acid and the iron. Like blackish brown stain? Put in excess iron into the pot, wait a couple of weeks had use it just like any water based stain. Like a redder stain? Put in excess vinegar, wait a couple of weeks and use like any other water-based stain.
I use the stain to raise the grain on my rifles. Each rifle gets 3 or 4 applications of stain, followed by a good rub down with OOOO steel wood. On the carving I use a brush made from hemp rope to burnish the stock wood.
5 years ago I built my 54. On that rifle I had a pretty even balance and it came out reddish brown. On my nephew's rifle he wanted more of a walnut color so I added iron to the pot. By the time his rifle was ready for staining it was turning the wood a nice chocolate brown. On my brother's rifle he wanted a redder stain so I added vinegar to the pot and by the time his rifle stock was ready to stain the stain was turning maple wood a nice reddish brown. Paul's rifle was in there too. He liked a dark stain so he got extra iron. Now that I am building another one for my self I will add a little iron to brown it some from what I had for my brother's rifle.
Since modern nails are often made from manganese steel, these days I hve started getting my iron from old barbed wire that I find up on the farm.
I have been using the same pot of stain ever since I built my first rifle over 20 years ago. (I built a couple disasters quit for 15 years and have now gotten back into it.) A couple of the kids' guns were stained with commercial stain. I didn't like the way they came out but the kids don't care - it goes bang and makes lot's of smoke. But on the adult rifles, for family and friends, I use the cider vinegar and iron stain. Besides, all of the old time gunmakers had plenty of vinegar at their dispoal, they did not have to have it shipped in from England.
I hope this answers your questions. And yes, I was a chemistry major in college.
If your mixture has just turned a dull gray it could be because of alloying elements other than iron. I started with rusty nails from an old barn. Now i am using bits of old barbed wire for iron. It could also just need more acid (vinegar). My pot of stain has been brewing for over 20 years! I end up adding vinegar or hard, sour cider every so often to make up for evaporation. It looks like coffee. You could also add some hydrogen peroxide (H2O2) solution, obtained from the pharmacy. Hydrogen peroxide is know as a bleaching agent because it oxidizes pigments, changing their color. When iron is exposed to hydrogen peroxide solution the H2O2 breaks down to H2O and O, free oxygen. that free oxygen should combine with the iron to produce iron oxide. But iron oxide is not very soluble in water. The ferric ion needs a cation like the nitrate or the acetate to keep it in solution. When the solution of iron ions is applied to the wood and the solution dries, the positively charged ferric ions are left with out a negatively charged nitrate or acetate and combines with atmospheric oxygen during the drying process. The key is in finding a negative ion that keeps the iron in solution but leaves once the stain is aplied. the acetate works well because of its high vapor pressure at room temperature.
Yes there is a bit of a difference between white vinegar and cider vinegar in that white vinegar is distilled, I think over mineral oil but I'm not sure, to refine out the compounds responsible for the tan color. As long as the acetic acid contents are the same the chemical properties will be the same. when I have to add vinegar to my pot of stain I go into the kitchen and get what ever is in the cupboard. If you REALLY like a red color you could probably use wine vinegar but the wood might turn out purple!
Ferric nitrate mixed with dilute acetic acid (vinegar) would probably produce a similar solution but I have never tried it.
Over the next couple of days I will try my hand at posting some pictures at theat photobucket site so you can see what I get with an iron/vinegar stain.