Especially when you're working on uneven land, or a poorly poured footer, there is a possibility that each layer of bricks (one layer, one brick "high" is called a course.) on a side is slightly higher or lower than the other. This is corrected by using the mortar.
Mortar comes in a bag, usually a 30-100lb one, and usually for under $4. I chose a gray mortar, because it makes the thing look old. Gray mortars are usually cheaper than white ones, but depending on sales, you might find a cheaper one near you...just make sure you use the same brand for the whole project, or you'll end up with a mismatched raised bed.
I divide my 30 lb bag into 3 10lb sections, using lidded buckets and cat litter pails. When you need it, mix with water to the consistency somewhere between toothpaste and pancake batter. Since I don't use it all at once, I can eyeball it and add more mortar or water as needed.
Recovered brick is thirsty, unless you've been keeping it underwater (and I don't recommend that) so be prepared for the mortar to become very dry while you use it. Add more mortar mix or water as needed to your bucket, and keep water on hand..
Normally, you would use a trowel to place a layer of mortar on the surface of your footer, then slide the bricks onto it, squishing some of it onto the edge. Because you're using found brick, which will have irregularities, I prefer to trowel mortar onto two sides of a brick, and then put it in place. Use a level to make sure the brick is not slanted (although a slight lean towards the center of the raised bed is good for water retention) and then place the second brick, and make sure it's not slanted AND it is at the same height as the previous brick. You can place more, or less, mortar, as needed, lifting bricks when needed and tapping them with a mallet when you only need a slight dip on one side. This will squish the mortar.
Using a finger (preferably gloved) or a trowel, remove excess mortar from the side. If, like me, your raised bed will be buried or hidden on a side, you can leave the excess mortar in place on those sides. It depends on how sloppy you want to be.
Wait about 5 minutes. Your brick will absorb the mortar and it will be the texture of damp sand. Now take a gloved finger, or a tool, and "shape" the mortar you have access to. Build it no more than 4 bricks high before letting it sit at least a day. I like to make my mortar joint (the layer of mortar, like frosting between layers of cake) about as thick as my pinky finger (~1cm) but when using reclaimed brick, you need to add or subtract mortar to make it level, so this is just an estimation.
I prefer to build up to one "wall" at a time. Depending on your style, you can lay bricks in any number of patterns. I used the most common one, called stretcher bond. This website shows several different bonds, and also the corners first, then fill in the middle, technique that is supposed to keep the wall from sagging. I lay the first course, then build the corners of one side, (2 of the four corners,) then build from one of the corners (constructing one more corner) then build the third side and 4th corner, then fill in the last section.
Because these beds are not bearing a very heavy load (just about 6 cubic yards of lightweight peat-based soil) it's okay to make it one brick thick. If you are making your bed more than 5-6 courses high, you'll want to make it two bricks thick to handle the additional weight. Next summer I should be making a more substantial pair of raised beds, and I'll take pictures of those in progress.
It really is like building with legos or other blocks. You probably know how to do it already. It's just a case of waiting between courses so you don't shift bricks once they are laid. In the next entry, I'll talk about things you can do while building to make the brick raised bed look old. The first and most obvious is to use reclaimed brick, with dents, dings and color variations, of course....