Cobalt blue is highly prized both as a pigment and as an unusual color for glass. In both cases, cobalt oxide produces the distinctive cobalt coloring. Most clear glass begins as a combination of a silica or sand, plus soda and lime. Glass makers add elements and minerals, such as iron or cobalt oxide, to derive different colors in the glass-making process. Cobalt is distinctive for its rich and deep intensity. Other blue glass may be lighter or darker than cobalt and is often created by adding copper to the mix.
Wash your glass bottles or shards in hot, soapy water. Protect your hands with gloves when handling glass pieces. Dry the glass carefully with a clean cotton cloth.
Work in a well-lit area. Spread the dry blue glass pieces or bottles on a white towel or white sheet on a flat surface.
Sort the blue glass from darkest to lightest, with the darkest pieces to the left and the lightest pieces to the right. According to the Society for Historical Archaeology, several shades of blue are darker and denser. Those darker shades most usually fall into the "cobalt blue" classification. Glass collectors and artists frequently classify the lighter, less dense blue shades, such as sky blue and cornflower blue, as part of the "sapphire blue" family.
Consult a color-spectrum chart or a paint pigment swatch to match your various blue shades of glass against a medium true blue. Compare your blue glass pieces to the true blue color on the chart. Those glass pieces or bottles darker than medium may be classified as cobalt. Glass lighter than medium may be classified as sapphire.
Examine glass shards with a magnifying glass to detect identification marks that may be embossed on the glass. Some commercial products, such as Vicks, Noxzema skin cream and Phillips' Milk of Magnesia, use true cobalt glass in their packaging. According to collectors in the North American Sea Glass Collectors Association, pieces of those products wash ashore and are valued by collectors. Some collectors comb the beaches to add pieces of cobalt blue sea glass, which they believe to be the best type of sea glass they can acquire.