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
5%
2%
86%
5%
37
flagthe Netherlands
8%
2%
82%
5%
34
flagAustria
14%
7%
78%
0%
28
flagGermany
10%
6%
76%
6%
275
flagSingapore
9%
0%
75%
15%
32
flagSwitzerland
4%
12%
75%
4%
24
flagCanada
7%
8%
73%
10%
56
flagUnknown
12%
3%
72%
11%
54
flagNigeria
11%
9%
71%
7%
53
flagthe United Kingdom
13%
6%
70%
7%
361
flagAustralia
15%
2%
69%
12%
119
flagFrance
14%
10%
67%
7%
28
flagPoland
21%
5%
66%
6%
24049
flagthe United States
11%
5%
65%
15%
987
flagGreece
0%
6%
65%
27%
29
flagMalaysia
14%
8%
60%
16%
48
flagRepublic of Ireland
18%
9%
57%
15%
33
flagIndia
21%
10%
51%
15%
790
flagthe Philippines
21%
5%
50%
21%
75
flagPakistan
31%
7%
37%
20%
82