  Flux growth involves dissolving the components of the target compound in an
appropriate flux at high temperatures and then crystallizing under
supersaturation controlled by cooling or evaporating the flux. A refractory
crucible is generally used to contain the high temperature melt. The reaction
between the melt and crucible materials can modify the composition of the melt,
which typically results in growth failure, or contaminates the crystals. Thus
one principle in designing a flux growth is to select suitable flux and
crucible materials thus to avoid any reaction between them. In this paper, we
review two cases of flux growth in which the reaction between flux and
Al$_2$O$_3$ crucible tunes the oxygen content in the melt and helps the
crystallization of desired compositions. For the case of La$_5$Pb$_3$O,
Al$_2$O$_3$ crucible oxidizes La to form a passivating La$_2$O$_3$ layer which
not only prevents further oxidization of La in the melt but also provides [O]
to the melt. For the case of La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$, it is believed
that the Al$_2$O$_3$ crucible reacts with NaAsO$_2$ and the reaction consumes
oxygen in the melt thus maintaining an oxygen-free environment.
