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
6%
0%
90%
3%
31
flagGreece
0%
0%
88%
11%
17
flagGermany
12%
6%
73%
9%
100
flagSwitzerland
0%
13%
73%
6%
15
flagUnknown
12%
3%
72%
11%
54
flagthe Netherlands
13%
4%
72%
9%
22
flagCanada
6%
10%
71%
10%
46
flagSingapore
10%
0%
71%
17%
28
flagthe United Kingdom
14%
6%
69%
7%
295
flagNigeria
10%
12%
69%
7%
39
flagPoland
21%
5%
66%
6%
22379
flagthe United States
11%
5%
65%
15%
887
flagIndonesia
22%
0%
61%
16%
18
flagAustralia
18%
2%
60%
18%
76
flagMalaysia
12%
10%
60%
17%
40
flagRepublic of Ireland
18%
4%
59%
18%
22
flagIndia
21%
8%
53%
14%
594
flagthe Philippines
20%
4%
53%
19%
62
flagthe United Arab Emirates
23%
5%
47%
23%
17
flagPakistan
30%
9%
36%
20%
65