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
7%
7%
80%
3%
84
flagGermany
10%
6%
76%
6%
641
flagSingapore
9%
0%
76%
13%
43
flagUnknown
12%
3%
72%
11%
54
flagSwitzerland
11%
8%
71%
6%
45
flagFrance
10%
10%
71%
7%
38
flagthe United Kingdom
14%
6%
70%
7%
457
flagCanada
8%
10%
69%
12%
75
flagAustralia
16%
2%
68%
12%
139
flagPoland
20%
5%
67%
6%
25528
flagthe United States
11%
5%
66%
15%
1098
flagRussia
12%
7%
65%
12%
40
flagRepublic of Ireland
14%
7%
61%
16%
42
flagNigeria
16%
9%
58%
13%
125
flagMalaysia
12%
8%
57%
21%
57
flagIndia
22%
10%
49%
14%
972
flagthe Philippines
22%
4%
48%
21%
87
flagPakistan
31%
9%
36%
21%
99