Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
- This specification provides guidance on removing lime mortar deposits from both clay and calcium silicate bricks using hydrochloric acid and water.
Material composition and method of manufacture are the major differences between clay and (sand-lime) calcium silicate bricks.
Clay bricks are composed mainly of sand and clay, and formed by molding, drying and burning them into shapes. Some characteristics include the following:
- Compact texture.
- Generally free of cracks, lime, stones and pebbles.
- Uniformly burnt.
- Calcium silicate bricks are composed of a sand-lime mixture compressed into precise, uniform shapes. Some characteristics include the following:
- Smooth, fine textured and light in color
- Steamed, not burned.
Read "General Project Guidelines" along with this specification. These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO). The guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
NOTE: Chemical products are sometimes sold under a common name. This usually means that the substance is not as pure as the same chemical sold under its chemical name. The grade of purity of common name substances, however, is usually adequate for stain removal work, and these products should be purchased when available, as they tend to be less expensive. Common names are indicated below by an asterisk (*).
Hydrochloric Acid (30-35%):
- A strong corrosive irritating acid.
- Other chemical or common names include Chlorhydric acid; Hydrogen chloride; Muriatic acid* (generally available in 18 degree and 20 degree Baume solutions); Marine acid*; Spirit of salt*; Spirit of sea salt*.
- Potential Hazards: TOXIC; CAUSTIC TO FLESH; CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS; FLAMMABLE.
- Available from chemical supply house, drugstore or pharmaceutical supply distributor, or hardware store.
- Clean, potable water.
- Drying Materials: Clean natural fiber rags.
- Garden hose and nozzle.
- Stiff bristle brushes (non-metallic).
- Wood or plastic scraper.
- Provide adequate wash solutions (i.e. water, soap and towels) before starting the job.
- Whenever acid is used, the surface should be thoroughly rinsed with water as soon as its action has been adequate. Otherwise it will continue etching the masonry even though the stain is gone.
3.02 ERECTION, INSTALLATION, APPLICATION
NOTE: DO NOT TRY MORE THAN ONE TREATMENT ON A GIVEN AREA UNLESS THE CHEMICALS USED FROM PRIOR TREATMENT HAVE BEEN WASHED AWAY.
Stone and Clay Bricks:
- If possible, remove large pieces of mortar deposits using a wood or plastic scraper.
- Wash down the affected area with a diluted solution of hydrochloric acid (1:10 by volume).
- Thoroughly rinse the surface with clean, clear water and allow to dry.
Calcium Silicate Bricks (or sand-lime bricks):
- CAUTION: THIS TREATMENT SHOULD BE USED CAUTIOUSLY, AS ACID MAY DAMAGE THE SURFACE OF SOME CALCIUM SILICATE BRICKS.
- Lightly abrade the surface using a brick of the same color.
- Wash down the affected area with a solution of hydrochloric acid (1:20 by volume).
- Thoroughly rinse the surface with clean, clear water.