What is the ruling on praying upon a brick whose interior is impure but exterior is washed?

Chapter on Praying with Impurity and Other Matters

Al-Mughni

Book of Prayer

Book 3 · Issue 3 · Bab 8

Open in Qurani

Primary text

If one prays upon a brick whose exterior has been washed and purified, the act is treated as praying upon a pure mat that is laid over impure ground.