Stone cleaning

Minerva has developed many different techniques for the sensitive removal of surface pollutants. All projects are different and no dogmatic rules apply, but we prefer to use of a range of techniques including poulticing and timed intermittent mist sprays. For larger jobs we find the best way to clean materials such as terracotta, Bath stone, brick and even wood conservatively is with the Jos/Torc system.

Buildings that are cleaned without proper care can store up problems for the future. An example of poor practice (right) is this entrance porch that was saturation-cleaned by water booms that were left on for long periods. This method, commonly used in Bath and Bristol, leaves the structure too clean, removes the patina of age and, more worryingly, causes ‘blooming’ by the mobilization of soluble salts, especially around the stone joints, which will lead to rapid decay of the stonework. When cleaning brick, for example, the fireskin is easily damaged and once damaged the decay of brickwork can be rapid.

The JOS technique works with a mixture of water (up to 10 gallons per hour), low air pressure and fine, inert abrasive powder. Together these form a swirling vortex, which when fired onto the contaminated surface, removes unwanted elements such as grafitti, certain types of paints, carbon deposits, dirt and scale from a wide variety of materials. A range of JOS nozzles are available for most tasks. The standard nozzle will quickly and efficiently clean most surfaces. A micro nozzle can be used for more detailed work and when matched with a soft abrasive powder such as talc will not harm the surface. The JOS system is chemical free and environmentally friendly, which makes it safe and ideal for city centre use.

Another method of cleaning that we employ is the DOFF System of 'SuperHeated Water' The DOFF system is different from other 'steam' or 'hot water' systems currently available and should not be confused with them, as it is actually 'superheated' water that exits the heat-resistant nozzle, not just hot (70-90), or steam. The desired pressure can be adjusted by the operator and is independent of temperature settings. Temperature can be increased at anything up to 150c if necessary. The Doff is not suitable for old limewashes, delicate or decaying stonework but can be particularly useful for the removal of oil-based paints and emulsions.