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How To: American Clay Plaster Compression

By March 4, 2019 No Comments

How To: American Clay Plaster Compression

If you’ve been to one of our American Clay classes, odds are you have a pretty solid grasp on how to prep a wall and apply the clay. But the next step takes careful timing, and an understanding of how the clay “communicates” with you. This step is called “compression”, due to the fact that on a molecular level, you are compressing the clay molecules together after most of the water escapes during drying.

Ideally, after applying you would want to begin compression at a very specific point in the drying process. We refer to this point as “leather hard” because, obviously, the clay feels like leather. It is dry enough to hold its shape, yet wet enough to still be malleable with a plastic trowel. However, some jobs are bigger than others and maybe you don’t have the time to wait for it to dry to leather-hard to begin compressing. If you have let your clay completely dry before compressing, that’s fine. Follow these steps to rehydrate your clay and compress it into something beautiful!

Step 1:

Rehydrate your clay using a spray bottle and cold water. Spray your wall enough to where the clay quits absorbing the water, usually about 10 sprays in one concentrated spot. This is how you restore your clay to leather-hard. After spraying and before compressing, touch it with your hands. Does it feel a little bit like leather? Great! Is it still too hard and dry? Spray a little more water. Does it feel soggy and do your fingers leave prints in the clay? Let it dry out a bit. Move on to another spot.

Step 2:

Okay, its back to leather-hard. Next, grab a plastic trowel to begin compressing the clay. Why not metal? Read down to the end of this post to find out why. Holding your plastic trowel at about a 30 degree angle, drag it across the clay while applying the same amount of pressure you would use to push a car door open. In this photo, we are pushing the trowel to the right, not pulling it to the left. The side of the trowel that is off the wall should be the leading side, otherwise you will scrape your clay off the wall. The first few drags will sound like they are scratching. This is where listening to the clay comes in handy. As you drag in one spot more and more, the scratching sound will lessen and eventually it will sound smooth. The more you drag your trowel in one spot, the more you will need to flatten it against the wall to compress it completely. This is a majority of what clay compression is. When you have a spot compressed and feeling smooth, move on to another spot and repeat these steps.

Step 3:

In some areas, you might come across small spots where the clay has cracked a little bit. Don’t panic, this is normal and easily repairable. This often happens when there is too much moisture between the first and second coats of clay. The moisture needs to escape, leaving behind small cracks in the areas it was able to. When the area is leather-hard, use the curved edge of your trowel to apply focused pressure on the cracked area. The focused pressure will help to move the clay a little bit in order to seal the crack again. All fixed? Great! Keep on going, dragging your trowel against your leather-hard clay until it sounds a feels smooth.

So, back on the subject of why we prefer to use plastic trowels for compression instead of metal: Some clients have asked why we apply the clay with metal and compress with plastic. While it is absolutely possible to compress with a flexible metal trowel, we have found that metal trowels leave behind a burnish that can’t be undone. Even when clients ask for a metal burnish, we compress with plastic first and then go back over it with metal trowels to burnish it. More often than not, client’s opt for a non-burnished finish, meaning we have to use plastic. And even if a client wants a burnished finish, we go incredibly light with it. A little goes a long way with burnishing.

Compressed clay, non-burnished

Compressed and burnished clay

And that’s it! While some clay plasterers prefer sponge compression, we almost always go with trowel compression in order to get a smooth finish. Either way, you will have beautiful walls. It all comes down to personal preference. Check back next week for an outline on how to do sponge compression!

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