Question

The colour of blue glass is due to the presence of oxide of:
A. Chromium
B. Cobalt
C. Gold
D. Silver

Answer 1

Hint: Cobalt is a very intense colouring agent and very little is required to show a noticeable amount of colour.

Complete step by step answer:
Cobalt glass—known as "smalt" when ground as a pigment—is a deep blue coloured glass prepared by including a cobalt compound, typically cobalt oxide or cobalt carbonate, in a glass melt.Cobalt is very intense glass colorant and very little required to show a noticeable amount of blue colour. Hence oxides of cobalt are used to give blue colour to the blue glass.

Hence the correct option is B.

Additional Information:
Moderately ground cobalt glass (potassium cobalt silicate) called "smalt" has been historically important as a pigment in glassmaking, painting, pottery, for surface decoration of other types of glass and ceramics, and other media The long history of its manufacture and use has been described comprehensively. Cobalt aluminate, also known as "cobalt blue",[5] can be used in a similar way.
The earliest known example of cobalt aluminate glass dates to a lump from about 2000 BC in ancient Mesopotamia, very possibly intended for use as a pigment, it was rare until the modern era. Cobalt oxide appears as a pigment in Egyptian pottery about five centuries later, and soon after in the Aegean region, and this is the pigment normally known as smalt. In paintings, smalt has a tendency to lose its colour over a long period, and is little used today. However, when used in ceramics for underglaze decoration, it keeps its colour well, and is the main blue used in blue and white pottery from a wide range of dates and areas, including Chinese blue and white porcelain from the Yuan and Ming dynasties, Renaissance Italian maiolica and Delftware.

Note:
Cobalt glass plates are used as an optical filter in flame tests to filter out the undesired strong yellow light emitted by traces of sodium, and expand the ability to see violet and blue hues,similar to didymium glass.