What is the amount Of alum you can use for Pickleing onions?!

Question:

What is the amount Of alum you can use for Pickleing onions?

Answers:

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Pickled Onions
10 oz medium (1-inch) boiling onions, peeled (about 16)
1 1/2 cups malt vinegar
1/2 cup water
3 tablespoons packed dark brown sugar
2 tablespoons pickling spice
1 (1-inch) piece fresh ginger, peeled and sliced preparation
Cover onions with water in a 2- to 3-quart heavy saucepan, then bring to a simmer and cook 5 minutes. Drain and transfer to a clean heatproof jar. Simmer remaining ingredients in saucepan 5 minutes, then pour over onions. Cool completely, uncovered. Chill, covered, 1 week to allow flavors to develop.
Cooks' note:
Pickled onions can be chilled up to 3 weeks.

I do not find even one recipe that calls for that ingredient. Sorry.

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Do you have a class reunion coming up or something?

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