When I read old texts, I often see them write out multiplication explicitly instead of using the square symbol. For example, in Riemann's paper "Über die Anzahl der Primzahlen unter einergegebenen Grösse" in a section he wrote:
... so kannman $\log\xi(t)$ durch $$\sum\log\left(1-\frac{tt}{\alpha\alpha}\right)+\log \xi (0)$$ausdrücken. ...
I also see this in Jacobi's book "Fundamenta nova" where he define the complementary modulus $k'$ as $$kk+k'k'=1$$
In the other sections of his book, if the quantity inside the square is complicated he would use the square symbol $^2$
Nowadays nobody is going to write $x^2$ as $xx$. So my question is, when did the consensus of using the symbol $x^2$ as $xx$ start to establish?