You can reuse vinegar from pickled eggs although it's not recommended as the vinegar loses some of it's acidity and flavor after it's been used to pickle the eggs.
This lose of some of the acidity can impact the quality and safety of any new batches of pickled eggs or pickled foods.
When the eggs pickle, the eggs absorb some of the vinegar's acidity and leaves the remaining vinegar less potent for pickling new foods.
The amount of salt you use for pickling depends on the food you're pickling and the type of vegetable and whether or fermenting it.
For pickling firm vegetables you should use a 1.5–3% brine, or 15–30 grams of salt per liter of water.
For pickling radishes, onions and cucumbers you should use a 5% brine, or 50 grams of salt per liter of water.
For pickling olives you should use a 10% brine, or 100 grams of salt per liter of water.
To calculate the amount of salt needed for your pickling.
You can weigh the water you need and add 2% salt.
You can mix 4 cups of water with 2 tablespoons of salt.
You can use 0.035 grams of salt per 100 grams of vegetable.
You don't want to use too much salt or too little salt as too much salt in the brine for pickling can kill off all of the bacteria and prevent fermentation.
Too little salt can allow harmful bacteria to survive.
The vinegar that is best for pickling is distilled white vinegar.
Distilled white vinegar is the best for pickling due to it's neutral flavor which does not affect the color of the vegetables and provides a clean, tart acidity ideal for preserving pickles.
Apple cider vinegar can also be used for some pickling and it will add a slightly sweeter taste to the pickled foods.
The reason why you boil vinegar when pickling is to help dissolve the salt and the other spices in the brine.
Boiling the vinegar when pickling also activates the spices flavors and achieves a more even distribution throughout the pickling liquid.
Essentially boiling the vinegar creates a hot brine that can better penetrate the vegetables and also ensure the proper preservation by killing any unwanted bacteria which may be present.
The high heat from boiling the vinegar also helps eliminate any harmful microorganisms which can spoil the pickled foods.
And boiling the vinegar allows the flavors of the spices and the herbs to fully meld together into the vinegar.
And depending on the food or vegetables being pickled, boiling can also soften the texture and it can be desirable for certain pickles and pickled foods.
Some pickling methods, such as for delicate vegetables will use a cold brine where the vinegar is not boiled.
Most quick pickling recipes involve the boiling of the vinegar mixture in order to achieve a fast pickling process.