I have a problem a yellowish white substance leaching out of my exterior slate stair. Once dried, it is a real pain to get off. I was thinking about using an acid wash, but was afraid of the stone delaminating. I tried a light sanding, but that made the stone smooth. I was also thinking about a high-pressure water gun, but that also may ware the stone.
Any suggestions about how to clean my slate, and how to properly seal it once clean?
Replies
Let me take stab at this one. A little geology from 30+ years ago.
Slate is the result of the compression of a sedimentary rock known as shale. Shale is produced when very fine silt is deposited with organic materials in a shallow basined sea. These shale and slate layers are often associated with coal and oil deposits.
Because of the composition of the original sediments that eventually became the slate on your stairs there may be organic compounds remaining in the stone that, when exposed, leach out.
The description of a "yellowish white" substance reminds me of sulfur compounds, which should be soluble in a mild acid solution. Try 5:1 hydrochloric acid or 10:1 sulfuric acid.................rinse thoroughly.
Sealing exposed stone has always been a problem of longevity and complete even coverage. And sealing does not exclude the continued "flowering" of the chemicals under the sealant. Experimentation is in order.
Lastly, your concern about delamination of the slate.......................that is a natural weathering process that slate undergoes as it breaks down when exposed to the elements. Slate erodes. Most commonly the physical method of frost fracture and heaving effects slates. What you may not be able to determine when you purchase slate is the tightness and thickness of the bedding planes.
So the spalling off of layers of slate is a weathering process. Specific geologically defined beds of slate have better (slower) weathering characteristics than others. Consulting local applicators or inquiring about specifications for the quarry's slate products may give insight into durabilities.
Enough said...............pick an inconspicuous spot and experiment. Maybe there are some spare pieces somewhere....set them out to weather.
Good Luck..........................Iron Helix
IH,
Just a word of warning. When I had a similar situation, I tested and it seemed like no problem. Then when I did the whole area, a few of the stones reacted differently than the rest. They were etched by the weak muriatic acid and look like c__p! I have yet to get them all the way back.
Good Luck, John