I am pleased to report the success of the pyrotechnics project! Arri alchemists have finally managed to apply the last of the dyes we were working on, and we just now had a successful showing for the first time in the years this research has taken. As such, I can guarantee you will be receiving a large batch for the Ignius Solstice this year, just as you requested during your last visit.
As for the mechanism behind this invention – the one you asked me to elaborate on during our prior correspondence - these boxes are equal parts magic and alchemy. The design starts by first choosing the color dyes an explosion should be. As a basic rule, dyes closer to the center of a box will detonate first, and time delays will increase proportionally to the distance of a dye from said center. There's a bit more to color selection than that, but it's something that can't be shown on paper.
Now the boxes themselves have arrays of runes carved all throughout the interior and the bottom. Each box has a ribbon on top to unleash the blast when it is ready and also has an injection point on the inferior surface for the essential fire magic to be placed inside. Usually, that magic is injected just before a show, as these boxes become highly volatile after preparation is completed. Essentially, though, the design of the box, from the array of alchemical dyes to the configuration of runes and circles placed inside, allows us to create near infinite possibilities in the realm of magical optics with little more than magical knowhow and specially crafted dyes. You'll see what I mean at the Ignius Solstice this year, and I believe you will be very pleased.
-Episcia Firequake, Master Alchemist and Arsonist
P.S. Who put that arsonist title there?! I swear I'll blast them up into the sky if I catch them!