I feel that I should say that not all the relevant information on the subject of gun bluing and browning is written down even in that Bible on the subject "Angier's." Also formulas passed from person to person weather verbal or written get changed a little and it is in certain cases understandable because we are trying to duplicate formulas in some cases two hundred years old.
Lets take the original "Niedner" formula, the subtle discrepancy is in the "wire nails" because when the formula was first used nails be them wire or any other form where made mostly from Wrought Iron and not as todays, made of steel. The formula works far better if bits of wrought iron or pieces of cast Iron are dissolved in the Acid.
Now the vexing subject of "Spirit of Nitre" or sometimes written "Spirit's of Nitre", then sometimes you come across "Sweet spirits of Nitre." The first are the same chemical substance, though the third is not in fact it was a very popular druggist's medication until it was withdrawn from public use caused by a couple of unfortunate accidents. Though the one thing they all have in common is the active ingredient "ETHYL NITRITE" this is also not available over the counter for public use, because it is part of the illegal drugs scene and is known as "Poppers." Now there are a number of schools of thought about Spirit of Nitre and why it is added to vintage bluing solutions Ethyl Nitrite is so unstable that it can only exist in Alcohol as a percentage. It is there to give the mixture a pleasant smell though I am not too sure about this. Though the one thing that people are not made aware of is that very often it is included in a formula containing Mercuric Chloride, the reason for this is simple Mercuric Chloride will not dissolve properly in water YOU MUST MIX IT WITH ALCOHOL FIRST and add it to the water, I have never seen this mentioned in how to make the formula up hence Spirit of Nitre may just be included just provide the needed Alcohol and a pleasant smell! So over the years I have used high percentage Alcohol and not bothered with Spirit of Nitre and found things work fine.
Now one personal little thing I do before I started browning Antique muzzle loading guns with English "common twist" or sometimes known as "skelp" barrels, to improve the contrast between the Iron and Steel I darkened the Iron with Tannic Acid first by rubbing the barrel all over a couple of times with an Acid soaked cloth this action also works well on Damascus barrels. This darkening of the Iron gives a far finer contrast between the Iron and Steel after browning. Though I will add it does not work for all Iron types but it is such a simple thing to do so it is worth trying.
