You need a temperature of about 1000°C to fire bricks, but more is better to get the moisture out. From cursory googling around it seems that a solar kiln wouldn't cut it, unless perhaps a very large solar collector was used... which upon further cursory googling yielded this:

https://helioscsp.com/concentrated-solar-power-for-high-temp...

https://cordis.europa.eu/article/id/421794-the-next-solar-re...