Blanks could be produced by fusing two separately cast sheets of glass, or by dipping the base glass into a crucible of molten overlay glass during blowing. The most famous example of a cameo from the early period is the Portland Vase. Although occasionally used in Roman cameos, the earliest prevalent use of shell for cameo carving was during the Renaissance, in the 15th and 16th centuries. Before that time, cameos were carved from hardstone. The Renaissance cameos are typically white on a grayish background and were carved from the shell of a mussel or cowry, the latter a tropical mollusk. In the mid 18th century, explorations revealed new shell varieties. Helmet shells ("Cassis tuberosa") from the West Indies, and queen conch shells ("Eustrombus gigas") from the Bahamas and West Indies, arrived in Europe. This sparked a big increase in the number of cameos that were carved from shells. Conch shells carve very well, but their color fades over time. After 1850 demand for cameos grew, as they became popular souvenirs of the Grand Tour among the middle class. Classically the designs carved onto cameo stones were either scenes of Greek or Roman mythology or portraits of rulers or important dignitaries. In history, agate portrait cameos were often gifts from royalty to their subjects. These antique cameos, some more than 2000 years old, are either displayed in museums or are in private collections. Many modern cameos are carved into layered agates. The layers are dyed to create strong color contrasts. The most usual colors used for two-layer stones are white on black, white on blue, and white on red-brown. Three-layer stones are sometimes made. The colors are usually black on white on black. The layers are translucent; this allows the artist to create shading effects by removing material to allow the background layer to show through. This way a very realistic, lifelike quality to a figure can be achieved. For example, thinning the top black layer on a three-layer stone changes its color to shades of brown. Removing material from the white layer creates shades of blue or grey, depending on the color of the base. The majority of modern agate cameos are carved with the aid of the Ultrasonic Mill. This is a process where multiple copies of a master design can be produced very quickly by pressing a master die onto the agate cameo blank. A film of diamond slurry is used to aid cutting and the die vibrates ultrasonically in a vertical motion.