Please pay heed to what the others have told you.
senorkevin wrote:What is the difference between Saltpetre and Cure #2?
Saltpetre is pure potassium nitrate. Cure #2 is a blend of table salt (sodium chloride), sodium nitrite, and sodium nitrate. Cure #2 is mostly table salt. The percentage concentrations of the nitrite and nitrate depend on the source of the Cure #2. As with your All-Purpose Curing Salt, you need to know what the concentrations of the nitrite and nitrate are
in the batch you buy. As with your Cure #1, the concentration of nitrite and nitrate will vary depending on the source. How much you need depends on the concentrations of nitrite and nitrate in the Cure #2, and what kind of cured meat product you are making.
Instead of cure #2 can I use saltpetre?
Not directly, no. As others have stated, you CAN use saltpetre as your source of nitrate, but
you would use a much smaller amount of pure potassium nitrate to achieve the same concentration of nitrate in the meat. If you do so, because the amount of saltpetre you use is so much smaller, you would need to dissolve the required amount in some liquid beforehand to ensure that it is distributed evenly throughout the meat.
I think I reduce it by 5 if using saltpetre but not sure.
No. The amount you would substitute depends on the concentration of nitrate in the particular Cure #2 you purchase. In the USA, standard Cure #2 contains less than 1/20th the amount of nitrate found in saltpetre. In the UK, Cure #2 contains a slightly different concentration, but again, less than 1/20th the concentration of saltpetre. It also depends on what kind of product you are making, because different kinds of cured meat products have different safe maximum amounts of nitrate, based on how they cure and how long they cure.
To further complicate things, the concentration of sodium nitrite in Cure #2 is usually (always?)
higher than the concentration of sodium nitrate, and the maximum amount of nitrite allowed in cured meat products is
lower than or the same as for the nitrate, so the amount of Cure #2 you can use in a given meat product really depends on the concentration of sodium nitrite in the Cure #2, and the maximum amount of sodium nitrite allowed in the cured meat product. This is because sodium nitrite is more toxic than sodium nitrate or saltpetre.
So, before ANYONE can tell you how much Cure #2 you need, you need to know the concentration (percentage content) of sodium nitrite and sodium nitrate in the Cure #2 you buy. It also depends on what you are making. I know you want an easy answer. The answer is NOT simple.
We use Cure #1 and Cure #2, instead of pure sodium nitrite and saltpetre, because the nitrite/nitrate are evenly distributed throughout the salt, and because the amounts of Cure #1 and Cure #2 are easier to measure than the very small quantities of pure nitrite/nitrate needed. Since the old days of using saltpetre, we have learned that saltpetre and nitrite are toxic, and in the case of nitrite, can be lethal. It is NEVER a good idea to use the amount of saltpetre found in old recipes. It is ALWAYS a good idea to recalculate the amount of nitrite/nitrate needed based on modern science and currently established limits. So, you need to know how much nitrite/nitrate is allowed in the kind of product you are making, how much you need for the method of curing you are using, how much your Cure contains, and how to calculate the correct amounts.
I'm glad you are asking these questions. People are wording their replies strongly because you are working with chemicals that are beneficial in the right amounts, and potentially lethal in the wrong amounts. You need to know what you are doing when you are curing meat. If you do not cure the meat properly, botulism can sicken or kill you. If you do not use the right amount of cure, the nitrite and nitrate can sicken or kill you.
I am a food historian. I, for one, would love to see the old recipes you have referred to. There are home curers, hobbyists, who think that, because people ate cured meat their entire lives that was cured with the old recipes and survived, that they did so with no ill effects. This is not true. We know from their bones and historical accounts, that people who ate improperly-cured sausages and meat died of botulism, and suffered the effects of long-term nitrite/nitrate toxicity, including, but not limited to erectile dysfunction, infertility, birth defects, lowered bone density, and nerve damage. While you can use the rest of a recipe unchanged, you should ALWAYS recalculate the amounts of nitrites and nitrates (an salt) needed for a proper cure.
I apologize for the lecture, but you need to understand this, Kevin. I'd much rather repeat things you know, than risk you plowing ahead without knowing.