"Email ombrant" is a pottery-decorating technique developed in France in the 1840s at the Rubelles factory by Baron A. du Tremblay. A picture or design would be etched or stamped into the pottery, and then a transparent colored glaze would be poured over it, so the surface would be smooth after firing, but the glaze would be different depths depending on how deep the design had been etched. The design appears shadowy, hence the name ("ombre" is shadow in French, and "émail" is enamel).

In 1851, examples of this technique received a gold medal of merit at the Crystal Palace exhibition in London. The Wedgwood company purchased all the molds and designs from Rubelles in 1872 and started making pieces with the technique; in 1877 is when the OED shows the phrase first used in English (without the accent).

Sources: the Oxford English Dictionary online, http://www.msstate.edu/Fineart_Online/Backissues/Vol_10/
faf_v10_n02/opportunities.html http://www.antiques-and-arts.com/majolica/majolicaemailombrant.htm http://duras.uchicago.edu/cgi-bin/quick_look.new.sh?word=email