What was added to this glass to make it blue?

ink
lead
cobalt
magnesium
Powdered cobalt glass, called smalt, has been historically important as a pigment in painting and pottery. It is colored to a deep powder blue hue using cobalt ions derived from cobalt oxide. Cobalt aluminate can be used in a similar way, and is now more common, often known as "cobalt blue".
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Answers by country

ink
lead
cobalt
magnesium
answers
flagSweden
4%
2%
88%
4%
42
flagAustria
7%
8%
78%
4%
94
flagthe Netherlands
10%
4%
78%
6%
47
flagGermany
10%
5%
76%
6%
724
flagSingapore
9%
0%
76%
13%
43
flagUnknown
12%
3%
72%
11%
54
flagFrance
9%
11%
70%
9%
44
flagthe United Kingdom
14%
6%
69%
7%
477
flagCanada
8%
10%
69%
12%
75
flagPoland
20%
5%
67%
6%
25881
flagAustralia
17%
2%
67%
12%
140
flagSwitzerland
17%
6%
67%
6%
58
flagthe United States
11%
5%
66%
15%
1120
flagRussia
12%
7%
65%
12%
40
flagRepublic of Ireland
15%
6%
61%
15%
44
flagMalaysia
12%
8%
58%
20%
58
flagNigeria
15%
10%
57%
14%
127
flagIndia
23%
10%
50%
14%
986
flagthe Philippines
23%
4%
47%
21%
88
flagPakistan
31%
9%
36%
21%
99