Stephani Stephenson on tue 5 may 09
This question of large sculpture, leads me to think about architectural c=
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lays, and the first=3D20
'layer' that comes to mind is brick and brick clay, as Vince was mentioni=
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ng brick.=3D20
Generally when I think of brick , I am thinking of structures which are/=
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were built of clay=3D20
rather than clad with clay. Aside from the fact that brick, in these inst=
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ances is load=3D20
bearing and must have good compressive strength, the other difference, is=
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that much=3D20
more clay is needed if the whole wall, rather than the outer few inches i=
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s made of clay.=3D20
Clay is heavy. Brick structures, and pavements used huge amounts of clay.=
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Until the=3D20
development of the steamshovel at the end of the 19th century, only shall=
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ow digging of=3D20
clay deposits was possible. So cities and towns traditionally relied on =
=3D
local source for=3D20
brick clay. Local clay varies. Even clay from the same pit varies. So b=
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ricks and brick=3D20
quality varied. firing and kilns varied. More on that later. Bricks are =
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also made=3D20
differently, molded , pressed and extruded. different methods yield brick=
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s of different=3D20
densities.
With the steam shovel, deeper deposits could be mined. The building of =
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railroads and=3D20
canals allowed for more cost effective transportation and concentration o=
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f brickmaking=3D20
and other architectural ceramics industry.
Note on firing:
There is not single set firing temperature for brick.
Source says brick must be fired hotter than 900 C (1652 F) to be conside=
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red a durable=3D20
brick with a degree of vitrification. So that is , understandibly, the ba=
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re minimum, same=3D20
as pots pretty much.
in fact , the range is given as a "Fusing temperature from 1600 F to 2700=
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F.=3D20=3D20
Just a wee bit of wiggle room, wouldn't you say?
.Building and facing brick are typically fired from 1600-2200, firebrick=
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2400-2700.
Tilemaker and brickmaker lore says that a lengthy firing improves strengt=
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h. I have an=3D20
idea why, but=3D20
I would sure like to find out more definitive information about that, as =
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in how long and=3D20
what degree of improvement is gained by lengthening firing cycle....) ge=
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nerally , for=3D20
brick, I see mentioned 40-150 hours... but don't know what kind of mass t=
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hey are talking=3D20
about and if it matters. and of course , brick firing temps can have huge=
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variations,as in=3D20
clamp firing, or can be closely controlled.=3D20
Water and freeze thaw:
Water absorption in bricks is measured by the taking the average of 10 b=
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ricks, though=3D20
some sources say 3. Engineering bricks are dense , strong, molded under =
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high pressure=3D20
and fired carefully so they meet thresholds for strength and water absorp=
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tion. Damp=3D20
proof course brick are also dense bricks with a low water absorption.=3D20
Both of these types of brick would be used for the base of freestanding =
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walls or retaining=3D20
walls to prevent rising damp.
What is low in brick world? Less than 4.5% of their dry weight.
BTW This source claims there is no direct correlation to water absorption=
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and durability.
Absorption below 7% usually indicates good frost resistance.
(Quote :" the destructive affect of frost is due to the 9% increase in vo=
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lume that occurs=3D20
when water at 0 degrees C is converted to ice at the same temp)
One could use only more vitreous bricks for ultimate resistance to water =
=3D
and moisture,=3D20
but it would generally be cost prohibitive..so the finest, strongest bri=
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cks are used where=3D20=3D20
absolutely nothing else will do, and other grades used where they suffice=
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.
Frost resistant quality brick are used only where the detailing cannot el=
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iminate or=3D20
minimize water saturation.
SO, much of the genius in the longevity of brick buildings has to do wit=
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h the skill of the=3D20
brickwork, i.e . "the detailing of the brick work is a more important fac=
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tor than the brick=3D20
itself."
What is detailing?
Drainage, seepage and water movement are a consideration in construction =
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and joinery.
Joints and brick are covered strategically where appropriate, (overhangs=
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, gutters, etc.)
Different types of bonds are utilized (bonding is the type of arrangement=
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or patterning of=3D20
the brick) ,=3D20=3D20
the bedding, the joinery...all these direct the moisture in an intentiona=
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l manner,=3D20
throughout a given structure.
the mortar is also a factor . Some types of mortared joints, such as we=
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athered joints,=3D20
help prevent moisture from entering, (by manually compressing the mortar=
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close up to=3D20
the brick and by sloping the joint so that the lower brick lip is protect=
=3D
ed and water is=3D20
sloughed off.)=3D20
Moisture also EXITS a building via the mortar.. think of water wicking t=
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hrough brick and/=3D20
or terra cotta, into the mortar then back into the air via the mortar. M=
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ortar helps the=3D20
breathing of the building.
Because of this, the mortar , as opposed to the brick, tile or Arch. cer=
=3D
amics, will exhibit=3D20
damage first..
and usually only in the top 1/2 to 3/4 inch.
Repair and replacement of mortar, is relatively easy, the joints are simp=
=3D
ly repointed ,=3D20
..much much easier than replacing brick , or entire sections of brick.
Brick structures are designed for movement. Expansion joints are utilize=
=3D
d for thermal=3D20
variation as well as settling , earth movement, etc.=3D20
The source also notes that all bricks expand eventually , due to moistu=
=3D
re, though the=3D20
degree depends on the=3D20
brick..and interestingly, the most 'succeptible' is right after it comes =
=3D
out of the kiln, when=3D20
it reabsorbs atmospheric moisture. standard recommendation is not to in=
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stall any brick=3D20
for first 48 hours apres kiln.
. finally
in brick world there is a "weathering index" which for any given locale i=
=3D
s the product of=3D20
the average annual number of freezing cycle days and the average annual w=
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inter rainfall=3D20
in inches
if you have an index of 50-500 or over 500, you should use brick which ca=
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n handle=3D20
'severe weathering' (Grade SW) on vertical or non vertical surfaces, in =
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contact with earth=3D20
or not, because a brick is likely to be saturated with water and freeze, =
=3D
thus a high and=3D20
uniform degree of frost action and weathering is desireable.=3D20
if you have less than 50, Moderate Weathering (MW) is OK because the bri=
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ck is not likely=3D20
to be permeated, even though it will be exposed to freezing temps, thus a=
=3D
'moderate and=3D20
somewhat non uniform resistance to frost action is acceptible. (the exept=
=3D
ion might be to=3D20
non vertical surfaces in contact with earth.)
=3D20
OK in all , this I haven't even gotten to tile, architectural ceramics, o=
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utdoor sculpture,=3D20
clay bodies etc.=3D20
Except to say that what will work depends on what you need.
claybody, firing temp, climate, location and positioning of work...all ma=
=3D
tter.
i guess better luck next time as I am surely near the max on this post.=3D2=
0=3D
my source ofr a lot of the brick info was=3D20
"Brickwork :Architecture and Design" Andrew Plumridge and Wim Meulenkamp
Stephani Stephenson
Revival Tileworks
http://www.revivaltileworks.com
\=3D20
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