Remove the tamarind from the shells and discard any strings or veins attached to the pods.

Place the tamarind pulp in a large bowl. Sprinkle with the baking soda, then pour the hot water over the pulp. Let soak for 30 minutes, or until softened.

Use your hands or a spoon to mash the softened pulp into the water, separating as much of the pulp from the seeds as possible.

Pour the mixture through a fine-mesh sieve into a clean bowl or pitcher. Use the ¾ cup water to rinse and press the pulp through the sieve.

Stir in the remaining 2 cups water to dilute the concentrate.

Add the sugar and stir until completely dissolved.

Stir in the lime juice, if using.

Refrigerate until well chilled, then serve over ice.