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Glass Blocks Tips and Tricks, continuedSpacers. Premanufactured plastic spacers are available, but I just use pieces of 1/4-inch by 3/4-inch screen molding cut into spacers that are a couple of inches longer than the block is wide. I set two pieces of screen molding on top of each block in the course that has already been laid, butter the bed and head joints for each successive block, and lay it into place (Figure 4). That ensures a consistent 1/4-inch bed joint.

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Figure 4. Wood spacers made from screen molding ensure consistent bed joints.

After the mortar has thickened (usually about 1 1/2 to 2 hours, depending on temperature and humidity), I go back and gently remove the spacers. But I don't throw them away -- I have a whole collection of them, and the older ones are worn down enough that I can use them to set a minus-1/16-inch joint. If I have any doubt about whether the spacers are ready to remove, I gently push on the mortar with my finger to see if it's firm. Using this method I can go up three courses before I have to stop and wait for the mortar to set (Figure 5). If it's a waterproof installation, like a shower wall, I'll go back and pack mortar into the holes left by removing the spacers.

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Figure 5. Three courses of block can be laid before waiting for the mortar to set.

Plumb, level, and square. As you work your way up the wall, check each block with a torpedo level to make sure it's sitting true (Figure 6).

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Figure 6. Every block gets trued with a torpedo level (left), while the 4-foot level measures the overall plumbness of the wall (right).

I also periodically check the overall true of the wall with a 4-foot level. If it's getting out of line, you can knock it back into shape with a rubber mallet (Figure 7). Don't be shy with this tool. Glass block is amazingly tough, and you can really beat on the block until the mortar has set up. You probably have three to five hours to make adjustments, depending upon the temperature and humidity of the room. Remember that the mortar will set up faster the closer to the ceiling you get, especially if you are working in a space that has temporary heat.

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Figure 7. A rubber mallet is the tool of choice for making minor adjustments.