The perfection of form and purity of line which distinguish the work of Kjeld and Erica Deichmann are as familiar to lay lovers of pottery as they are to the seasoned critic. From the Deichmanns' kiln come pieces which rival the best in fine bone chinas. Today, eager buyers flock to their farm-studio near Saint John, N.B., to choose new pieces almost before they're cool. Many years of patient experimentation with a variety of glazes and designs have brought the Deichmanns to the front ranks of pottery makers as, under their sensitive fingers, each piece becomes, in its own way, a collector's item.