Why do some Limoges plates look slightly translucent?

The transparency is due to the porcelain body and its thinness. Hard-paste porcelain, made from kaolin and fired at high temperatures, can become glass-like in structure. When the rim of the plate or the wall of the cup is thin, light can pass through the rim, which is why collectors love the soft glow. Sheer doesn't automatically mean antique, and it doesn't always mean the highest quality, as some modern pieces are thin, and some older pieces are heavier. Look at translucency along with glaze quality, weight balance, crispness of molded details and brightness of painted work. If the decoration looks smooth and printed, transparency alone will not increase the value very much. If the painting is skilled and the gilding clean, transparency can add to the appeal.