How is gold jewelry that has been left in a safe for twenty years and not previously paid Zakat on, purified, knowing that its weight is 400 grams?

You are obligated to pay Zakat (definition) on the gold you kept after your mother's death for 20 years. Zakat is not waived due to forgetfulness. You must hasten to pay Zakat for the past years by giving out a quarter of a tenth (2.5%) of the gold for each year that it completed a full year (hawl) and reached the nisab (definition) (85 grams or more). Since the amount of gold is 400 grams, 10 grams are to be given out for the first year, and then a quarter of a tenth of the remainder for the second year, and so on.