I recently received the following questions regarding slaking and sand ratios in lime plaster (Natural Hydraulic Lime):
As always, we appreciate your response. We finally ordered the lime,
it is arriving this morning. We are scheduled to start tomorrow. Just
lining everything up today. Some final technical questions.
In the formula you call for 4 different grains of sand. In my local
Home Depot I can only find “play sand” and “Masons Sand. I suppose
I’ll have to go to a better source to find the variety of sand. When
I do, what is the proportion of different sands that I need? Probably
this information is on the video, yes?
I have purchased a large mixing tub from a masons supply house
thinking this would be ok to slake the lime/sand mixture in. The
after thought was that you had suggested creating a troff with 6 mil
plastic. The question is, do I have to slake all the lime/sand
mixture over night? It would make sense in this case to have the
large troff constructed. If on the other had I can mix in batches
then I would rather not build the troff and mix in the large tub.
Here’s my response:
You do not have to slake the lime at all if you don’t want to. It makes a better plaster if you do, but will be absolutely fine if you do not. Just be sure to mix for a minimum of 20 minutes before you use the material. If you do slake the lime, you will want a fairly big trough to put it in as a single mortar mixer load produces a lot of mud. If you plan to slake all the plaster, then you will need to hold at least four or five mortar mixers full at one time for 24 hour slaking.
The sand itself can be play sand as long as the sand is not washed. The same is true for mason’s sand. The key is that you have at least four different size grains of sand. The scratch and brown should be fairly coarse sand and the final coat should be fine sands. I think most of the sand at Home Depot will be washed and graded and therefore no good for you. You can call sand supply yards and ask for their play sand and confirm the washing/grading as mentioned above. The mixing ratio of the grains is not that crucial; however, an even mix ratio is great if it can be achieved.