Outdoor Weather resistant ceramics

Recently Pistrucci Artworks completed a restoration deck project. The homeowners had purchased a porous tile from Mexico that had been done in Majolica style and terra cotta clay.  In the pacific northwest we live in a very wet, cold climate and also have quick freeze – thaw weather conditions.  As a tile designer it is important to consider the application of the building materials as well as good design. 

Restoration Deck Project:

Deck Restoration Project
showing original design of the deck and layout for tile

Mexican terra cotta tile weathered

Weathered porous Terra cotta tile

Deck Restoration project

Pistrucci Artworks Porcelain Painted tile for Restoration deck project


Source: http://digitalfire.com/4sight/education/index.html

Outdoor Weather Resistant Ceramics

Section: Clay Bodies, Subsection: General


How can you be sure that the porosity of your fired ceramic ware is low enough to prevent freeze-thaw breakdown in the winter?


Almost all fired stoneware and sculptural ceramic has some porosity. Porosity (also called absorption) is typically measured by weighing a sample, boiling it in water, weighing it again, and calculating the percentage increase in weight. This means that it will absorb water into the surface. In climates exposed to freezing temperatures water soaked ceramic experiences a buildup of pressures within the material as ice formation continues (water expands 9% as it freezes). If the ceramics cannot withstand this then scaling and cracking occur. Over a period of years the material can completely crumble. Concrete is susceptible to the same problem, this phenomenon is called Spalling .

Examples of clay porosity (these assume that the clays are fired at a temperature appropriate for each, underfiring will increase the porosity)

Red or brown burning stoneware clays 0.5-4%
White or buff burning stonewares 1-3%
Porcelains 0-0.5%
Warp resistant sculpture clays 5-10%
Vitreous sculpture clays 2-5%
Red terra-cotta clays 10-14%
White or buff talc clays 8-10%

Thus, if climatic conditions demand it, outdoor ceramic installations must be able to survive the stresses of freeze-thaw .

Since any ceramic with more than zero percent absorption demonstrates water penetration it would appear that it is theoretically susceptible to freeze-thaw damage. For practical purposes however, this is not altogether true. The brick industry considers any clay having under 5% porosity resistant to freeze/thaw failure (regardless of its closed and open porosity).

Closed and Open Porosity

More porous ceramic actually has both absorbency and porosity (technically they are not the same). A fired piece will naturally absorb a certain amount of water to fill the pores (open porosity ). However more porous clay matrixes also have capillary networks that normal soaking does not fill (closed porosity ). This auxiliary network allows fired ceramics to survive freeze-thaw because the expansion of the water has somewhere to go. The addition of finely dispersed cellulose fiber to a clay body could theoretically improve the capillary network.

It is possible to perform a simple test based on the principle that a sample of fired ceramic boiled in water will absorb more moisture than one that is simply soaked. This is because for the former, the entire network is filled, for the latter only the pores. This test compares the cold soaking absorption or open porosity (C) of a clay with its boiled absorption or closed porosity (B) . The structural ceramic industry requires a C/B result of less than 0.78 in order to pass CSA and ASTM specifications for outdoor use. If you are buying clay, your supplier should be able to measure and provide this information for you. They will want to know the temperature and rate-of-rise of your firing.

The C/B test procedure is defined in detail in the FORESIGHT software Test Information database. The procedure uses 10 mm thick by 25 mm wide by 120mm long fired test bars and defines a 24 hour soak and weigh, then a 5 hour boil and weigh.


Dry Weightv1
The weight of a dry test specimen of the fired clay.
24hr Wet Weightv2
The weight of a specimen that has been soaked for 24 hours in room temperature water and wiped clean of all surface water.
5hr Boiled Weightv3
The weight of a specimen that has been soaked for 24 hours and boiled for 5 hours and wiped clean of all surface water.


24hr AbsorptionC
This is a calculation of the C value for this test, that is, the absorption of a clay sample if soaked for 24 hours in cold water.C = (v2 – v1) / v1
5hr Boil AbsorptionB
This is a calculation of the B value, that is, the absorption of a clay bar if soaked for 24 hours and boiled for 5 hours.B = (v3 – v1) / v1
Saturation CoefficientS
This is a calculation of the C/B value, the cold water absorption divided by the boiling water absorption.S = C / B

The saturation coefficient S should be less than 0.78 in order to pass CSA and ASTM specs for outdoor use.

Sealing the Surface

Many products are available from building supply stores to seal the surface of concrete and masonry. These just soak in and harden to plug the pores. The use of these is standard practice in construction. So even if your fired ceramic does have a high porosity you can just seal all surfaces.


These relate to the problem of spalling.

In Bound Links

  • (Project)  Ceramic Tests OverviewEvery ceramic production facility should have some…

Coast Collective Art Classes: Photolithography on Clay

Photolithography on Clay

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Instructor: Angela Pistrucci
Date: May 19 Time:  Sun, 10-4pm Cost: $95
Class Code: 0513-AP-MMW   Registration Deadline: May 10Coast Collective 
3221 Heatherbell Road
Victoria, BC
V9C 1Y8
Ph: 250.391.5522
An easy method of transferring images onto  clay projects using a few simple materials. Learn how to create your  photographs and graphic arts by adding the dimensional quality of clay. Add  photographic/graphic images to

pottery, functional ware or personalize a  project. In this course we will be making a book  cover or photo cube. We will be slab rolling, making our own ink, designing,  reversing & transferring image to clay as well as applying a transparent  glaze. The project will be kiln fired and students will have to pick it up 2-3  weeks after the project is finished.

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Coast Collective Art Classes :Sculpture

Structure of the Head in Clay – Beginners

Sculpture 'Elle'
Instructor: Angela Pistrucci Date: Feb 22, 23 & 24 Time: Fri, Sat & Sun, 10 – 4 pm Cost: $225

Class Code: 0213-AP-MMW Registration Deadline: Feb 15

Coast Collective
3221 Heatherbell Road
Victoria, BC
V9C 1Y8
Ph: 250.391.5522

Learn the basic structure of sculpting the head in clay.  This is an excellent course for those who want to attempt a different art form  or practice 3d. Sculpture of the head is also an excellent technique to build  on drawing skills. In the workshop we will: Build an armature using 2 methods,  cone and dowel board. Golden mean and Facial proportions and beginner  sculptural methods for building the skull, Eyes, mouth, nose and ears.

Art Classes at Coast Collective

Making your own art supplies: Experimenting with Paperclay  & Molding Mediums

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Instructor: Angela Pistrucci Date: Jan 26 Time: Saturday, 10 – 4 pm  Cost: $105

Class Code: 0113-AP-MMW Registration Deadline: Jan 18
Coast Collective
3221 Heatherbell Road
Victoria, BC
V9C 1Y8
Ph: 250.391.5522

Art supplies can be expensive.  In this course you will learn how to make art supplies out of inexpensive  materials found in your cupboards. Easy methods to create mediums that will add  dimension and texture to art forms. We will cover many recipes such as any  color chalkboard paint, recipes for encaustic, paperclay and molding paste (  many other recipes and painting mediums to be available to the class- bring  binder). In this course we will create poly fresco painting and airdry  paperclay for relief. The technical applications for specific projects and  material selection for specific art projects will also be discussed. If you are  itching to add dimension to your artwork or practice with texture and dimension  this is the course for you.