Sandstone
Sandstone has been quarried locally along the front range since at least the 1880s. In 1912 the bottom fell out of the sandstone market, probably due to the wide-spread use of concrete, but it revived quickly in the early 1930s with another surge of popularity in the late 1940s after World War II. It is a relatively cheap and available -- lying in shallow layers near the surface along the front range. It has a high compression strength and it is relatively hard.
The color ranges from purple to a deep dark red to almost white with the red colors found near Boulder and the lighter colors as far north as Loveland. The deep rich reds are more scarce as are the buffs streaked with iron oxide (stain faced).
University buildings have sandstone walls that are laid up flat with the fractured face exposed beyond a common vertical wall line of mortar. The ends of each stone are clipped back to give a convex or “lip out” effect, also known as “shadowing”. Front range masons have seen this as a distinct “ University of Colorado Style.” Wall patterns can be bold with short large “jumpers”, or delicate with long small stones and few “jumpers”. See the graphic sketch and discussion in the book, “Body and Soul”, for further detail.
The graphic wall sketch illustrates the standards used by the University of Colorado at Boulder for laying up sandstone walls. Some further comments are listed below.
- Several patterns exist on campus which are unique to the particular building mass and form. A large expanse of wall often requires a larger and stronger appearance than a small expanse of wall. A small building or one with walls broken up by windows or other materials patterns will often have a smaller pattern of laid up stone.
- Overall color of stones used should be compatible with buildings adjacent to the new. The mix specified by the project should be obtained and assembled at the site.
- Mortar color should be gray, but warmed by mixing in a minimum amount of red coloring.
- The mortar between the stones should be a vertical plane parallel with the building substrate. All stones project out from that wall line to a greater or lesser extent. (See sketch)
- Mortar joints should be as consistent in thickness as possible. This is especially true for width of vertical joints. Some walls are specified for thinner joints than others.
- A pattern of non-vertical joints from 15% to 20% in number should be included in the wall pattern. The slant should be about 30 degrees. Too much or too little slant is not acceptable. The joints with adjacent stones should be as consistent as possible.
- Stones should have a good textured face, and iron oxide stains are encouraged. Flat uninteresting stones are just that – boring. Limit the number of smooth faced stones in a wall.
- Except where specifically called for by the specs, stones should be generous in length. Every stone should be well over a foot in length, and can be as long as practical. In all cases, the minimum ratio of stone height to length is 1:3. Anything less is a plug and not permitted. The thickness of stones is set by the specs, but generally should not exceed 7” or 8”. An over-abundance of very thick stones is undesirable. Thin stones can be as little as ½” to ¾” if permitted by the specifications.
- Those stones that project well out from the vertical mortar line are to be clipped back at each end. This is unique to the Boulder campus. Projections should be uniformly created in a pattern across the wall so that there are not expanses of wall with few projections and others with too many. These projections create a uniform shadow pattern across the wall.
- Corners of sandstone walls are struck to a straight vertical line top to bottom. Each corner stone shall be trimmed to that vertical corner line.
- Where a window is set in a continuous wall, the stones that abut the window opening should be laid so that the pattern disappears into the opening. That is, stones next to the window can be shortened down to 1:3 proportion, but no less.
- The wall pattern is not started or stopped at an expansion joint. The wall is sawcut later and caulked according to project specifications.
- Other instructions may be provided by any members of the Project Team during the course of construction.
