On the achromatic number of the Cartesian product of two complete graphs

A vertex colouring $f:V(G)\to C$ of a graph $G$ is complete if for any $c_1,c_2\in C$ with $c_1\mathrm{\ne }{c}_2$ there are in $G$ adjacent vertices $v_1,v_2$ such that $f(v_1)=c_1$ and $f(v_2)=c_2$. The achromatic number of $G$ is the maximum number $\mathrm{a}\mathrm{c}\mathrm{h}\mathrm{r}(G)$ of colours in a proper complete vertex colouring of $G$. Let $G_1\mathrm{\square }{G}_2$ denote the Cartesian product of graphs $G_1$ and $G_2$. In the paper $\mathrm{a}\mathrm{c}\mathrm{h}\mathrm{r}(K_{r^2+r+1}\mathrm{\square }{K}_q)$ is determined for an infinite number of $q$s provided that $r$ is a finite projective plane order.