Lining your outdoor pond with clay offers the advantage of using a non-polluting material. However, natural clay pond bottoms often leak water if they are not properly sealed and can release clay particles that float in the water and make your pond cloudy. There are a few different options on how to properly line a pond bottom with clay and how to get the water to stay clear using low technology methods.
Dig out the pond to the desired depth and width. Natural clay may already exist as a thick layer under the top soil you dig out. Check the soil particles to see if your pond is on a bed of natural clay, as this will save you from having to buy it.
Compact the clay using the tread of a backhoe. If you need to buy clay, then put down a layer of clay about 5 cm (2 inches) thick completely covering the bottom and sides of your pond. If you have access to livestock, a low-tech compacting alternative is to lead a horse or cow over the clay layer; its weight will compact it.
Add two to three more layers of clay, compacting each layer after it's lain. If cracks form in the clay, simply fill them with more clay. You can use Bentonite clay as a filler if you already have a natural clay bed.
Fill your pond with water, and check for leaks. If there are leaks present, let the remaining water evaporate or drain it, and add another layer of clay, making sure to firmly compact it.
Refill the pond with water, and plant marine flora on the bottom of your pond. These will slowly filter the silt particles from the water in your pond, clearing up the water.
Try to use native fresh water marine plants for the bottom of your pond.
You may need to fill up and empty the pond a few times before using it for fish, swimming or aquaculture because of any heavy minerals or toxins that may leach from the clay material.