Obtaining magnesium oxide from magnesium hydroxide
Magnesium hydroxide decomposes at 332 °C (630 °F) :

$$\ce{Mg(OH)2_{(s)} -&gt; MgO_{(s)} + H2O_{(g) }}$$

So heat the magnesium hydroxide in a covered crucible. The released water will cause the particles of magnesium hydroxide to "explode" so you want to do this covered.