Phase transitions (melting, boiling) require heat energy, but the energy goes into breaking up the molecules so they go from tightly packed as a solid (frozen) to more loose as a liquid, to very loose as a gas / vapor. Heat is taken in from the fire, but the temperature does NOT increase.
So basically you have to keep the pot on the fire for a longer time to melt the ice THEN bring it up to the temperature where boiling happens and the steam forms to pressurize the cooker.
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