Drain the oysters and reserve the liquid. Add the oysters in a food processor, process until finely chopped. Transfer the oysters into a small saucepan. Add the reserved oyster liquid and bring to the boil. Reduce heat, cover and, simmer for 8 to 12 minutes. Remove the saucepan from the heat, add the salt and let cool completely. Drain the mixture through a fine sieve into the saucepan. Discard the pieces in the sieve. Measure the liquid, adding 2 tablespoons of soy sauce to each ½ cup liquid. Add another 1 tablespoon of soy sauce at the end and, bring to the boil over high heat. Reduce heat, gently simmering for 10 minutes. Allow to cool to room temperature and pour into a jar that has been boiled in the boiling water for 15 minutes. Seal tightly and store in the refrigerator until ready to use. It can be kept, refrigerated for approximately 1 month.