Date: Wed, 6 Aug 2008
From: "Marcus Flynn" <pyromasse(at)>
To: "Norbert Senf" <mheat(at)>
Subject: Cracking ?


At 10:06 PM 8/6/2008 +0200, you wrote:

Hi Norbert,
                    Two images I took today you may be interested in. Sent them in high res as they are details.

Briefly:  A black oven top slab. Flat. 4 inches thick. Andalucite based castable I am familiar with.

             Brought it up to temp gradualy over three days.  Then a medium- large sized fire of softwood in the evening while it was still hot from a fire that morning.

             Saw what looked like crazing in the soot on the surface of the slab. Wiped the soot away with a dty rag but couldnt see any cracks on the cleaned surface. See images.  Temp couldn't have been high as it didn't whiten up.

           Not too worried as it seems very superficial.

Have you ever seen this?
Any ideas what it is ?

Note: Weighed the water and vibrated the mix in the form . The further I go and the more I apply myself to making good castable the more frustrating it seems to get.

Hi Marcus:

That is very interesting, and the first time I have seen it.

So, let me offer some speculation:

- perhaps perfectly normal micro-cracks, that somehow the soot deposit mechanism at work on the surface has the effect of magnifying visually.

In automobile engine building, there is technique called "magnafluxing" for checking crankshafts, connecting rods, etc. for flaws. A strong magnetic
field is applied, and any surface microcracks tend to concentrate the magnetic field. When iron powder is dusted on, it reveals the cracks.

A quick and dirty test I use to check castings is to scratch them with my pen knife, or a screwdriver. If there is something seriously wrong with
a castable, particularly one you are familiar with, it will be noticeably softer.

You can also buy a 30 power handheld microscope at Radio Shack for about $20.00, that is very handy for surface inspection.

Have a good one ....... Norbert

Norbert Senf---------- mheat(at)
Masonry Stove Builders  
25 Brouse Rd.
RR 5, Shawville-------          
Québec J0X 2Y0-------- fax:-----819.647.6082
---------------------- voice:---819.647.5092