To perform a mortar glass block installation on 3" or 4" thick glass block, it is necessary and recommended that you use a Universal 3" & 4" Mortar Spacer..
RELIABILT Glass Block Spacers 3-1/8”x1/4” 10 to a pack For Use With Mortar/Grout for sale online | eBay
Concrete Block Spacer, Their Uses, Construction
Seves Mortar System Glass Block Spacers for 3 in. or 4 in. Thick Series Block (24-Pack) SPACERGB1 - The Home Depot
Concrete Block Spacer, Their Uses, Construction
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2mm Spacer for Pegasus Block | Seves Glass Block, Inc.
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Plastic Glass Brick Spacer, Size: 10 MM at best price in Thrissur | ID: 2851761132173
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Spacer for glass block
1/4" Spacer for 4" Block | Seves Glass Block, Inc.