I wasn't sure where to put this exactly but I know to use earthen plaster with cob for waterproofing so I put it here.
Anyway, we have a pond on our farm that is down in a ravine, basically, and we have a path and series of steps that lead down to it. About halfway down is a widened area that we cut a bench seat into the hillside which is a great place to sit and watch the water and be out of view of anywhere else. A private little retreat area. We don't want it to wash away, though, and thought if we could cob it and then use a plaster of some kind (rather than straight concrete) it would be a great place to maintain and maybe build a small patio and firepit near.
I've made cob before ... we build a cob bread oven that worked great ... so I know the formula for that, but I don't want to use yucky straight concrete to cover it. Is there a percentage of cement I can add to my cob mix that will make it waterproof/water resistant and help preserve the seat and steps? Or is there something else that can be done or added to help with this? I don't anticipate ever building a roof over this area as that would defeat the hidden quality of it and block the view of the stars. Thanks for your help.
Cob with 5 - 10% linseed oil gets really hard and waterproof (like concrete). You can add the mixture in a thin (less or about 1cm | 3/8") layer, or apply linseed oil to the drying cob for a few days (replacing water with oil).
Why cob and not wood? I imagine a thin (~2cm) wood planks would make a surface that isn't that cold to sit on.
I was wondering what direction you eventually took with this project. I am looking to create a large patio area that is not covered. I am replacing where wood use to be and cannot afford the lumber. I live in the Midwest of the United States where four seasons are present in a fairly humid area. Did you ultimately choose to use cob or did you add cement?
Sebastian Köln's idea sounds really good for a nice durable surface. I might try to experiment with something like myself eventually.
From the way you described your project, I have the impression that you might be dealing with storm water rushing across the site.
I was recently reading about the restoration of old stone bridges and the author talked about rushing water's amazing ability to find any little crack to get back behind the stone and erode the soil.
I'm sure you won't be dealing with a massive deluge like a bridge over a large stream, but any amount of water can cause erosion as it washes across a surface...So I would recommend a homogeneous water-resistant mix. I would definitely NOT recommend a shell of portland cement as it will only cause problems for the cob it is protecting, as when people try to 'stucco' an adobe house with concrete.
Lime would be the way to go, which you can make from wood ash. It is unique in that unlike portland cement it will chemically react with the clay in the cob rather than just be weakened by it. And you won't have a bland gray mass, but just a lighter version of your cob's natural color. Keep in mind, though, that lime takes a very, very long time to really do its thing because it relies on gas exchange to work rather than water, like portland cement.
For info on making lime from wood ash, search YouTube for 'Primitive Technology Pot Made of Wood Ash'. Wood species seems to be important for calcium content, so you might want to try different flavors to see what works best. The guy in the video just made basically a mortar mix with lime and sand, which would probably be the quickest way to tell how well your ash is doing.
Lime is supposedly pretty picky about firing temperature, but I think in the case of wood (as opposed to limestone), the minimum temperature to produce clean, light-colored ash would be a safe bet. Overfired, or 'burnt', lime, isn't as chemically reactive, but again I think you would only see that with the temperatures used with limestone.