As this is a natural process, dependent on the mineralogy of the stone, color change percentages cannot be assured and expectations should be discussed with the supplier prior to ordering material. The process can become apparent in as little as a few weeks or take six months or longer. The percentage of color change in these weathering slates can vary from quarry to quarry, or even within the stone of a given quarry. In the quantities present, these minerals are not deleterious and do not affect the life expectancy of the material. Historically, the terms “weathering” or “semi-weathering” were used to describe slates that would show various percentages of color change – to tones of buff, tan, and brown – slowly, over varying periods of time, as the slates were exposed to the elements.Ĭolor weathering is due to the presence of calcite and iron compounds, primarily fine-grained iron pyrite in the form of limonite. Even for these unfading slates, color change on a roof can take place as the result of environmental pollutants, most commonly in the form of high sulphur content acid rain. These slates more or less maintain their original color on the roof. Where slates were known not to have these “fading” characteristics, the slates were described as “unfading” or “non-fading.” In current usage, “unfading” is the most common reference for this color characteristic. In some cases, the color change and mineral makeup had no impact on the longevity of the slate on the roof, and in other cases, the color change was due to accelerated deterioration of deleterious minerals that would lead to a shorter life expectancy of these slates. Historically, the term “fading” was used to describe slates that, upon exposure to the elements, had a discernible, marked, and generally uniform lightening in color due to their mineral makeup. Weathering is used to describe slates that would show various percentages of color change slowly as the slates were exposed to the elements. Unfading slates more or less maintain their original color on the roof. It is important to note that in many cases, and in current usage, the terms “unfading” and “non-weathering” and “fading” and “weathering” are often used interchangeably. Slate can be known as “weathering” and “non-weathering”, or “fading” and “non-fading”. There are two color adjectives that have been used in the roofing slate industry that explain the change of color known as weathering and fading. In order to obtain the best blending of these colors, contractors should draw from several pallets at the same time when taking slate to the roof for installation. These color variations should be expected. One of the unique and aesthetic advantages of natural roofing slate is the subtle variation in color, shade, veining, and grain of each individual slate shingle on a roof. Chlorite produces green slate, hematite the purples, carbon the grays and blacks, and hematite and iron oxide the reds.ĭescriptions of color can vary widely from supplier to supplier, but generally, roofing slate produced in North America falls under the general color descriptions of black, gray/black, gray, green, gray/green, purple, variegated purple, mottled purple/green, and red. The color of a natural slate is derived from its chemical and mineral composition. Slate Colors and Slate Weathering Designations Natural slate has many uses which include roofing material, floor tile, countertops, wall cladding, blackboards, and even tub and shower enclosures. As a result, the sedimentary bedding plains of the original clay and shale are totally independent of the metamorphic grain of the resultant slate bed. Sediments accumulated on the sea floor were exposed to extreme pressures which lead to the chemical and mineralogical transformations you see today. During later geological upheaval, these sedimentary shale beds were lifted from the ocean floor, folded, and buried on the North American continent. These sediments accumulated on the sea floor for 150 million years, forming a bed of clay and shale several thousand feet thick. Natural slate is a microcrystalline, fine grained metamorphic rock formed 400 million to 550 million years ago, with its beginnings as sedimentary silt washed into ancient seas.
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