By writing with ink, Cael bypassed the limitations of his engraving skill. The ink, a simple soot-based liquid, was not as mana-conductive as the snake horn, but the latter partially compensated for it. As a result, when Cael poured his mana into the glyph together with ink, he didn't have to push as hard as when he wrote on the sand.
The mana again immediately tried to shape itself into magic, rudimentary and unstable, but this time Cael was ready. He focused his will on it and prevented it from being anything more than shapeless energy.
This took some of his concentration, but not all. Cael kept writing the next glyphs of the spell, and pouring mana into them as he went.
It worked. While Cael's own mana was filling the glyphs, natural mana couldn't get inside and shape itself into anything explosive.