| Granite defects and the tricks used to hide them |
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issue time:2006-06-22 13:51
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Natural stone can have defects and unscrupulous
suppliers sometimes resort to unethical steps to
hide blemishes. An article on the FindStone
website (www.findstone.com) gives an insight into
some of the problems which may be encountered
with granite.
Oiling and colouring
Many black granites are light blacks. To make
them temporarily look better and blacker,
industrial oil is rubbed on after polishing. The
oil evaporates after some months leaving the slab
looking very scruffy. Tip: To check this, use a
burning match to heat the surface of the granite.
If it has been oiled, the oil will rub off on
your finger.
Similarly, red oxide is sometimes rubbed onto red
granites to make them look redder and to fill
cracks. Tip: Rub the surface hard with your
finger and see if some red colour rubs off on it.
The article points out that, by wetting the sawn
surface (the back side of the polished piece),
you can see the true colour and nature of any
granite slab or tile.
Shade and grain variation
Slabs and tiles should be sorted to ensure
uniformity of shade and grain. Normally, any
colour can be sorted into three shades, while if
the grain varies between fine and coarse, the
tiles and slabs should also be sorted uniformly
by grain size. Tip: Sorting is simple - just lay
all the material on the floor and look from
different directions. The further you are from
it, the easier it is to sort.
If you intend buying the entire quantity of stone
that you need over a period of time, you should
ensure that the material will be supplied from a
specific quarry There will always be a variation
in material supplied from different quarries.
Colour patches are darker or lighter patches or
bands of single colours due to mineral
localisation. With black granite this occurs
as "oil patches"- areas that look as if oil has
been spilled on them. These are actually
permanent patches and you should pay less for
granites that contain them. Tip: After laying out
the material and viewing it from different
angles, it is easy to see which pieces have these
defects. Tapping with your fingernails or a spoon
may give a different sound on these patches when
compared to rest of the granite.
Sometimes two different grain sizes occur in the
same slab, giving the appearance of a double
colour. Material of this nature should also cost
less.
Once you have decided on which granite you want,
remember that the shade and grain should be
uniform throughout the entire batch and there
should be no colour patches. Do not accept the
statement "this is not possible because granite
is a natural material". If all the material is
from one quarry, it should be consistent.
Cracks and holes
You can test for cracks in the granite by pouring
water onto the stone, wiping it and letting it
dry. Water that goes into a crack evaporates
last. Some crystallic granites do not have a
smooth surface. The surface chips or there are
pinholes. Avoid this type of granite,
particularly for horizontal applications.
Staining
Some granites do stain. Tip: Test the material
before buying it with wet spices or oil left on
it for a few minutes.
Even if staining is not a particular application
concern, if the surface is going to be washed
frequently, you should check how long tap water
remains absorbed. Some granites change colour
when wet compared to their dry colour. As drying
is a slow process, the surface can look
unappealing during the interim period when part
of it is wet and part is dry.
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