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
flagthe Netherlands
8%
4%
82%
4%
45
flagAustria
8%
8%
80%
2%
75
flagGermany
9%
6%
77%
6%
534
flagSingapore
7%
0%
77%
15%
40
flagUnknown
12%
3%
72%
11%
54
flagFrance
10%
10%
72%
5%
37
flagthe United Kingdom
14%
5%
70%
7%
439
flagAustralia
16%
2%
68%
12%
136
flagCanada
8%
10%
68%
12%
74
flagPoland
20%
5%
67%
6%
25273
flagSwitzerland
10%
10%
67%
8%
37
flagthe United States
11%
5%
66%
15%
1075
flagRussia
12%
7%
65%
12%
40
flagRepublic of Ireland
14%
7%
60%
17%
41
flagNigeria
15%
8%
59%
15%
113
flagMalaysia
12%
7%
58%
21%
55
flagIndia
23%
10%
49%
15%
947
flagthe Philippines
22%
4%
48%
22%
85
flagPakistan
31%
8%
36%
21%
95