I wanted to touch on the topic of this sculptor again after working my way through the book on him “Christian Petersen remembered,” specifically I wanted to address the part of the book where the author mentions that the various terracotta works were “refurbished” and repaired or restored around the 1980s after “40 years of exposure to the harsh Iowa climate” there was deterioration to the terracotta she stated.
I find that very puzzling that these terracotta panels and sculptures were showing deterioration after only 40 years! I have had hundreds of pieces of architectural terracotta from the 1880s and 1890s that were on building facades in New York City for 90 to 100 years and every single one of them was just as clear and crisp and pristine as the day they were installed!
Other than coal soot staining, bird droppings etc., the harsh New York City climate coupled with acidic rain from coal burning and later automobile exhaust had absolutely no effect whatsoever on the terracotta, it did on carved stone however.
In thinking about this further, we know from history that clay (terracotta) pots, shards of them (from being smashed) made by the ancient cultures, the Incas etc found in archaeology sites, in digs found in graves and under the dirt, these did not deteriorate.
In fact, many may not be aware of it, but BRICKS are terracotta and I happen to have pictures of the perfect example of the point I want to make in this post today, here’s a couple of photos of an 1880s era brick storage building here that was built by the railroad to store dynamite in, the railroad is long gone, the depot was moved, the roundhouse was demolished many decades ago and all that remains is this little roughly 10 ft by 16 ft building, the roof long gone and a large tree growing up in the center of it.
The first photo shows the surface on the inside of one wall, you can see how many bricks but not all have practically disolved into dust like sugar cubes that became wet, especially on the lower courses, but you can also see the bricks inside the wall are mostly in good shape while many of those on the outer course disolved away to dust and are gone;
Compare now to the outside of that wall, these bricks have absolutely no disolving away at all;
What is the explanation for this you might ask, if you didn’t know what I know about these particular bricks it might be a mystery- one chalked up to that terribly harsh Iowa climate! and on the surface taken at face value that might fly, but… now we come to the fact that I know about these particular bricks- they were made by a brickyard here in Iowa, they made bricks for many buildings, what happened was they had poor quality control on their brick firing process and as a result there are many brick buildings around the state which have a similar phenomenon of totally disolved bricks right next to pristine intact ones, I don’t know any further details beyond that.
Here’s some explanations on this:
Unless a kiln is fired to the correct temperature to mature the clay in it, and unless that temperature is held to that temperature long enough to soak every item inside the kiln so they are that temperature completely all the way to their center, then you will wind up with a fired terracotta load that has extreme variances- everything from over fired to underfired, if the terracotta is underfired that is a major problem and will cause the fired clay to be very porous, soft and it will absorb moisture and water, spall and it disolve just like those bricks in the first picture did.
Mind you- all of those bricks in both photos came from the same maker, these bricks are all about 130 years old and exposed to the weather the same- but actually those facing inside were protected by a roof for a time early in their lives, the others were never protected and faired much better because those happened to have been properly fired!
Kilns back in the 1880s when these bricks were made were fired by burning wood, this of course was a very highly variable thing and the fires had to be closely watched 24/7.
Think of firing brick in terms of baking a cake which is familiar to everyone who eats, we all know if you don’t have the oven temperature set high enough, or the cake left in long enough according to the recipe, that the cake will be mostly cooked on the outside, but inside it’s still raw or uncooked, this is exactly what happens with terracotta in a kiln, if the temperature is not hot enough and/or if the terracotta is not left in long enough to absorb the heat completely to the center, it is exactly like putting a cake in the oven, setting it to 300 degrees instead of 375 as the recipe calls for, and taking it out after 15 minutes instead of 25 as the recipe calls for.
With all of this in mind we come back to Mr Petersen’s terracotta panels at the university and their deterioration after just 40 years, actually if there was enough deterioration by 40 years for people to notice it enough to spark refurnishing/repairs/restorations, then there was already signs of deterioration years before that people hadn’t noticed because it only affected a small area or the damage was small.
In going back to the book, we learn that much of this terracotta was made from “local” iowa clay, the earlier batches which had been donated were of an unknown clay which Mr Petersen and Mr Cox had to grind up grog to add to it and experiment with getting it right, this is in contrast to purchasing a read-made clay whose source and properties were known, documented and controlled by the manufacturing company who mined, refined, blended the clay according to their various recipies.
When clay is purchased this way it’s quality and characterists are known, documented, it’s proven and tested as well as consistant, Mr Petersen unfortunately didn’t have the luxury of being able to do this due to lack of funds at the university, so they made do with the materials at-hand which they were able to obtain.
Since the book says Mr Petersen was not well versed in firing large terracotta pieces (he was a sculptor) and Mr Cox was a ceramicist who knew about his kiln, but had never fired such massively sized works before, there was a lot of experimentation, furthermore the book details how they fired the clay to 2050 degrees which is just about what is called cone 3 temperature- a medium-fire temperature, and they had students sleep on a cot watching the kiln’s temperatures all night to make sure the firing went well.
What I believe happened is one or more combinations of these possible problems:
1) The clay may have been poor quality to begin with.
2) Variances in temperature of the firing.
3) Variances in the heat-soak- thicker ware and larger loads require more time for the heat to completely saturate all the way to the center of the clay- the raw cake inside analogy per above.
4) The clay’s maturation temperature was higher than they thought- again the raw cake analogy- the sculptures did not reach a hot enough temperature to completely vitrify the clay, the result is a porous terracotta that is like a sponge.
One panel they had to remake three times due to problems with it during the firing, I believe from the description of it this one was that very deep relief containing most of the two cow’s heads in the center bas relief panel- very deep relief, lots of mass.
I suppose another thought comes to mind too, and that is they seemed to have had to re-invent the wheel here with all of this which on one hand I find puzzling too, because making terracotta panels the size of those on the campus is hardly new- every architectural ornament firm in the country, and there were about 100 of them around the turn of the century- made pieces this big and bigger out of terracotta and fired them just fine- I’ve seen pictures of them. Obviously the architectural terracotta companies managed to get the fine art of mass producing pressed terracotta for building facades before 1890 down to a “tee” and by 1920/1930 they had maybe 50 years experience in this country alone at producing thousands of pieces like these a month.
I’m not sure why the Petersen/Cox team did not seem to have all of this information etc available to them to use and seemed to have had to start from scratch re-inventing what was already in daily commercial practice nation-wide by 100 firms for 50 years by then. Maybe there was a cut-throat atmosphere that eliminated any sharing of technical information due to competition and rivalry, still, it’s difficult to imagine 100 different firms each having to re-invent the entire pressing and firing process themselves by trial and error from scratch, so I don’t have the answer to that, but I believe I do have the answer as to why these sculptures were deteriorating after only 40 years.