  Many NP-complete graph problems are polynomial-time solvable on graph classes
of bounded clique-width. Several of these problems are polynomial-time solvable
on a hereditary graph class ${\cal G}$ if they are so on the atoms (graphs with
no clique cut-set) of ${\cal G}$. Hence, we initiate a systematic study into
boundedness of clique-width of atoms of hereditary graph classes. A graph $G$
is $H$-free if $H$ is not an induced subgraph of $G$, and it is
$(H_1,H_2)$-free if it is both $H_1$-free and $H_2$-free. A class of $H$-free
graphs has bounded clique-width if and only if its atoms have this property.
This is no longer true for $(H_1,H_2)$-free graphs, as evidenced by one known
example. We prove the existence of another such pair $(H_1,H_2)$ and classify
the boundedness of clique-width on $(H_1,H_2)$-free atoms for all but 18 cases.
