Read the back of a packet from the top: name, ingredients, net weight, best-before date. The roasting temperature appears nowhere. The figures are in the industry's own documents, where roasting and hiire temperatures sit on rows far apart. Because of those rows, one temperature looks like the border.
What follows asks whether that border lies in temperature, time, purpose or judgement, taking each in turn.
A line the thermometer cannot draw
Turn to the appendix and roasting and hiire sit on separate rows of a table. The border cannot be drawn on the temperature scale.
That appendix is not a document defining kinds of tea. It is a set of examples for sorting out who is responsible for a label, and it names neither the equipment nor where the temperature was taken. An old public trial called heating at a roasting-side temperature hiire. A study has also made hojicha at a temperature on the hiire side.
The sources show that the temperature is not the line, and no more. After temperature, the easiest difference to see is time.
The line is the stage the fire meets
Pour hojicha and the smell of fire rides up on the steam. The line is drawn not by time or temperature but by the stage at which the fire arrives.
Of the three, time shows the difference most readily. Laboratory roasting is counted in seconds, hiire in minutes or hours. There are long roasts too, though, so time alone cannot draw the line.
Purpose and judgement cannot be compared head-on, because they belong to different stages. That difference of stage was itself the line. Hiire meets aracha before it is finished; the roasting fire meets sencha and bancha that have already had their hiire.
The same fire aroma is marked down at the earlier stage and sold at the later one. The packet writes that fire as light roast or dark roast, and whether a shared standard lies behind those words is taken up in the next chapter. The sources place the line at the stage, and stop there. Time, purpose and judgement follow in that order.