Mottahedeh Rare Vintage Lace Cut Dish / Matching Platter hand painted floral 24K The Porcelain Art Of Mottahedeh Knows No Age By BARBARA GAMAREKIAN Published: April 6, 1989 AS guests linger over after-dinner coffee at the White House and diplomats sip cocktails across the street at the newly restored Blair House, they are surrounded by what they suppose to be priceless porcelains: glorious Chinese export bowls holding lilies and freesia; olive dishes emblazoned with the American eagle, serving as ashtrays; a classic Tobacco Leaf epergne, filled with fruits and candy. The ceramics are indeed beautiful - beautiful fakes. But Mildred Mottahedeh, the energetic, 80-year-old collector of Chinese export porcelains whose company produces these museum-quality reproductions, would cross swords with anyone who chose to employ such a disparaging term. Fake indeed. Good reproductions insure that works of art will be preserved for future generations. ''The originals are just