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Moravian College • Preservation Master Plan lime-based mortars. These hybrid mortars combine the quick set and strength of Portland cement with the flexibility and greater workability of hydraulic lime. The great advantage of these hybrid mortars is that they are mixed and cured in a similar manner to modern Portland cements. Most masons are familiar with these methods, which greatly improve the chances of achieving a successful repointing job. Natural cements and natural hydraulic limes have become available in the U.S. and should be considered for repointing historic masonry. Natural cements are produced from burning clay-rich limestone at temperatures that are lower than those used to make Portland cement. Natural cements tend to perform in a similar manner as Portland cement and hydrated lime mixes. It is imperative that natural cements be installed by a mason who is familiar with the product and the curing requirements. Natural hydraulic limes are also produced from burning clay-rich limestone at lower temperatures, but typically have more free lime than natural cements. Hydraulic lime mortars are more vapor permeable than cement-lime mortars, which aids water and salt removal within the masonry, and have better elasticity, allowing for building movement without cracking. However, hydraulic limes do require treatment after placement to ensure proper curing, which is vital for frost resistance. The choice of a contractor with experience using natural hydraulic limes is the key to a successful project. Good Repointing Practice Mortar should only be removed when it is absolutely necessary, such as when the mortar is unsound, cracked, eroded, or crumbling. Removal of mortar at all joints in an effort to achieve a uniform appearance is rarely necessary. Unsound mortar should be removed to a depth of 2.5 times the width of the joint, or to sound mortar, whichever is greater. Work should be performed using hand-held, non-power tools. Power tools such as masonry saws can easily damage masonry. In some circumstances, a thin saw cut may be run down the center of a horizontal joint with the remainder removed by hand. However, masonry saws should never be used on vertical joints. This work should only be attempted by skilled preservation masons. New work should match historic mortar joints in color, texture, joint size, profile tooling, and any decorative details, such as penciling. Where necessary, voids in bedding mortar must be packed with new mortar, and then repointed to prevent face loading of the masonry and consequent spalling. Masons should achieve visual continuity between surviving historic material and new patches. Mask grouting is essentially a cosmetic fix that applies a skim topcoat of mortar over existing joints. Not only does it hide any underlying existing mortar problems, it changes the appearance of the entire building. This practice should be avoided. Masonry Replacement In areas of extensive deterioration or missing features, limited replacement of the masonry units may be appropriate. Replacement should only be considered if the material is deteriorated-beyondrepair. Removal or rebuilding of a substantial portion of exterior masonry walls should only be undertaken when it is crucial to a building’s structural integrity. The replacement masonry should be carefully chosen to match the damaged masonry in size, color, texture, appearance, and physical properties. New work should match the existing profile, pattern, and coursing details of damaged sections and features. Avoiding the need for large masonry replacement is important; preventative maintenance and minimizing openings for equipment installation should prevent the need for later rebuilding. Replacement brick must be suitable for exterior applications. Brick that is stronger than the original brick should not be used because of water migration that results in spalling and damage to softer bricks. New work should match the existing bond (pattern within a row), course (pattern of rows), color, and size of bricks when replacing sections of a wall. Brick should always be toothed-in to historic brickwork. Locating replacement stone to match the existing stone used in historic masonry can be problematic. Many historically used stone types are no longer quarried for building stone, and it is difficult to find sources for the dimensional stone used in walls. Potential sources of stone may include salvaged stone or replacement with similar, though not identical, commercially available stone from other locations. Indiana limestone and slate are still commercially available. The homogeneity of Indiana limestone John Milner Associates • October 2009 • Chapter 9 • Building Guidelines • 292
Moravian College • Preservation Master Plan makes it a reliable replacement material. It is relatively easy to match the color and texture of historic Indiana limestone. Replacing slate requires careful matching to available materials, which can range from mauve to gray to green. 9.4 Metals Metals are found in various architectural elements, including handrails, light fixtures, and gates. The metals most frequently used in architecture are alloys containing lead, tin, zinc, copper, nickel, aluminum, and iron. Iron and its alloys, including steel, are particularly prevalent because of the moderate production costs brought about by technological breakthroughs in manufacturing in the late nineteenth century. Metal elements are inherently durable if properly maintained. Metals used in the production of doors, windows, and their associated hardware will be dealt with separately in the section on doors and windows. Typical Metal Conditions Architectural metalwork is present in historic buildings at Moravian College as entrance handrails and balustrades, lighting fixtures, plaques, gates, cupolas, finials, and fire escapes. The majority of this exterior metalwork appears to be in relatively good condition. Metal Inspection Architectural metalwork should be included in regular cyclical maintenance inspections. Metal handrails and gates should be inspected as part of entrance and access safety. Rooftop ornament should be inspected during roof surveys, particularly to ensure that they are properly anchored. Inspections should note any of the following potential signs of deterioration. • Loose handrails. • Loose anchors or attachments. • Missing rails or decorative elements. • Rust or rust staining through paint. • Cracking or other disruption at anchors and screws. • Cracking through soldered joints. Causes of Metal Deterioration Corrosion is the major cause of deterioration of architectural metalwork and is exacerbated by the presence of moisture. Corrosion can be caused by structural stress, electrochemical reaction with dissimilar metals, or corrosive environments, such as salt-laden water. It is accelerated wherever water collects against metal elements, such as at post anchor locations in masonry. Metals undergoing corrosion are slowly reverting to their natural ores, such as iron oxide. This process involves significant expansion of the corroding metal, which can cause extensive masonry cracking. Architectural metals can also deteriorate from mechanical failures, such as overloading or fatigue. For example, excessive use of handrails will slowly work sections loose from their anchors and disrupt the masonry at the anchor connections. The exposed nature of metal fire escapes can lead to corrosion and potential life-safety issues. Fire escapes should be inspected regularly. Metal Repair The architectural metalwork at Moravian College’s historic buildings can be maintained through proper surface preparation and application of protective coatings. Deteriorated paint should be removed using appropriate methods, including wire-brushing for non-decorative elements exhibiting light rust, or chemical paint removal for heavier built-up paint. Severe corrosion may require that entire sections of metalwork be removed to a shop for repair. The newly-cleaned metal should be immediately protected with a rust-inhibiting primer. Alkyl-based enamel paints are recommended for finishing iron alloys. Latex and other water-based paints are not recommended. Metal Replacement When metalwork components are beyond repair, replacement of the element with reproduction metal is a practical solution. Where new or replacement handrails are required, they should match the characteristic environment of the character area. Modern stock handrails are not appropriate for most historic buildings, though sympathetic modern installations can be found. Decorative light fixtures and finials should be reproduced to match the existing historic elements in size, thickness, and details. John Milner Associates • October 2009 • Chapter 9 • Building Guidelines • 293
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