A concrete tragedy
A few years back, when I worked for my friend Hank Kaminsky, I had the sad experience of unveiling a concrete tragedy. I still remember the sad feeling of disappointment and dashed hopes.
Hank had been working for several sessions over a period of weeks with a class of high school students to create a piece of sculpture for the high school's new sculpture garden. With Hank's help, the students had created a large caterpillar out of clay. Then he showed them the process of covering the clay with plaster and removing the clay to make a mold. Then, I presume the mold interior was coated with a release, and it was filled with concrete. For, I had not been involved with the project up until the final day, as Hank had to go out of town and he wanted me to help the kids chip the mold off of the hardened concrete sculpture.
When I first approached the concrete filled mold, I saw something that worried me. It looked like the concrete had been mixed too dry and had a porous, gravelly texture. As I demonstrated the technique of chipping the plaster with a very blunt chisel, my heart sank. The concrete was so porous that there was hardly any surface to the sculpture - it was terrible, an utter failure! And, since the plaster mold was a "waste mold" which cannot be reused, the caterpillar sculpting was lost forever.
Now, I knew the pathology of this particular failure. I had heard Hank pass on the wisdom his father had passed on to him: that concrete was stronger when it was mixed with less water. There is some truth to this, of course, if you are setting fence posts or something where the finish is not that important. But as with all "knowledge," if it is applied without common sense, it will lead you astray.
First, if someone says that it's not good to make concrete too wet, that's no reason to make the stuff so dry that it will hardly hold together! But most importantly one has to be adapting to the particulars of each situation one faces.
Since that time, I have had lots of experience casting concrete into all sorts of molds and forms, and let me tell you, one has to work hard to get good surfaces on concrete castings. It takes a good flowing mix and a lot of poking, vibrating and beating with hammers to get concrete to satisfactorily fill a mold. But if you really want perfection, it is best to carefully spread a topping mix (with no aggregate) on the inner surface of the mold, working it into every detail, before filling the mold.
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