At PCC we have encountered some problems caused by inexperienced operatives during stone cleaning and our experience dictates how we approach the contract. Here is a list of the common issues and our tick list on how to proceed.
Deep water penetration can cause this it is the crystallisation of soluble salts caused by the passage of water, including rainwater. It shows as white powder on the face of the stone or mortar pointing. It tastes of salt its quite safe, obviously don’t make a meal of it, try it. Wet the end of your finger, touch the white fluff and then the end of your tongue. This is very unsightly and difficult to get rid of. The only treatment is a light washing and a soft brushing to remove the surface staining as much as possible. Bear in mind that water activates it so use it sparingly. It is probable that it will diminish after each winter as the rain water brings out the salts near the surface, and eventually it will disappear back into the stone.
Scouring or Gouging.
Caused by non -use of tips on the lance it is permanent and not treatable. This is in effect like sand blasting with water and results in literally stripping away the stone almost like a chisel.
Inappropriate use of chemical cleaners which are driven deep into the stone pores by the high-pressure lance. Common cleaning materials may contain acid or bleach or a high alkaline base. This can cause etching and spalling of the stone face. If not stabilised quickly they cause irreversible damage. Some types of stone are particularly susceptible such as marble granite and limestone.
Grout or Pointing.
Any loose or defective pointing which may not be visible or apparent before work commences is affected by the high-water pressure and simply falls out sometimes in long lengths. The only remedy is to have the whole affected area re-pointed. Its perhaps not as bad as first thought after all if the pointing is becoming loose it will have to be replaced at some point and exposing such areas ensures a longer life without repairs afterwards.
Standard Check List
- Have a good look at the type of stone and the condition of the pointing before commencing. If necessary advise client.
- Never exceed 1000psi pressure
- No zero tips, use only the fan type
- No bleach or acidic chemical cleaning agents
- Keep the tip min 13” (30cm) from the wall face
- Use “stone soap” ph neutral clearly labelled stone cleaner and rinse well.