Evaluation of a Terra-cotta Urn
Measuring approximately: H=73 cm x D=72 cm - 33 cm across mouth
Purpose of this evaluation:
Preliminary evaluation consisted of a physical examination/inspection with and without a 5X magnification on site with approximately 34 digital images being recorded. Digital images were then examined further and manipulated for contrast to better attempt understand what processes were involved and ongoing with the urn. Approximately 45 min. was actually spent with the urn with several more hours viewing digital files on screen. The observations, including possible causative agents/mechanisms, as well as proposed treatments were derived from the above process. Additional advice was sought from a professional colleague*.
The following questions were asked concerning the urn, and will be addressed below, each to the best of my ability.
1) "Can you determine why the urns are degrading and the cause?"
2) "What are your suggestions to stop this decay?"
3) "Can you estimate the age of the urn?"
4) "Do they look really old? New? Within 50 years?"
Condition, Main areas of concern:
Footed, two part terra-cotta urn decorated with egg and dart, gadrooning, patera, rosettes, leaves, and two medusa. The urn appears to have been painted or glazed overall with thin washes. There are other areas which appear to have been overpainted with one or more thicker coatings. Erosion of the surface has occurred for the most part, directly below this thicker coating/layer. Close examination of these areas reveal the presence of layers and the method by which they were applied; brush marks are quite pronounced at the interface of many instances.
Urn had been setting on a limestone pedestal in a garden for approximately15 months before the deterioration was first noticed. Location of urn is approximately 30 meters from the ocean on the north shore of Oahu.
Damage to the urn, generally will fall into the following categories: Breakage and Erosion.
Upper lip/rim section has been repaired with clay and blended (fig. 9-11) into undamaged areas; a blind crack extends down 15-20cm. below the repaired rim. (fig. 1 & 9) This crack appears to be stable (not continuing to travel), however, vibrations and expansion/contraction cycling may cause this to change There are numerous lesser cracks overall which will not be further addressed here.
It should be noted here that the foot/base does not appear to be securely attached to the main urn body and could possibly fall off while the urn is being lifted/moved unless precautions were taken.
Some areas on mostly upper surfaces are severely eroded. This surface erosion of the clay body extends down from the original (clay) surface several millimeters and in some areas deeper. Raised elements in the eroded areas have much of their edge profiles missing (fig. 5).
As previously stated, the erosion is quite limited to areas directly under or adjacent to, the thick applied coatings (fig. 2 - 8). Exfoliation of this coating is occurring as well as the erosion of the clay body beneath it. Where the coating becomes thinner or where only brush marks are visible, little if any erosion is visible.
To address question #1"Can you determine why the urns are degrading and the cause?"
This is likely due to several reasons, working in concert. As the total history of the urn is unknown, one is left to conclude from the present observable state of the piece. The two main conditions are: 1) The urn has had the application of a film forming coating, over certain areas only. 2) The surface of the coating and underlying clay body is exfoliating in areas that coincide with the coating.
The original reason for the application of this coating is speculative, however, it is likely that it was applied to conceal some sort of defect to the clay surface in these areas.
Soluble salts crystallizing within the clay body under the coating is likely the cause of the erosion and exfoliation seen, though there may be other processes involved that predate the application of the coating layer and may be exacerbated by it.
A quote from The Conservation and Restoration of Ceramics by Buys, S. and Oakley, V. (1993) London, Butterworths states:
Damage caused by soluble salts
One of the most damaging factors, in so far as porous pottery is concerned, is that of water soluble salts once they have been absorbed by the body of the wares, and this is especially true of the salts that tend to deliquesce at high relative humidities and then recrystallize during drier periods. It is in fact there crystallization that causes the damage, since during this process the newly forming crystals occupy a greater volume than the salt solution and exert enormous pressure on the fabric of the pottery. These may be sufficient either to cause the surface to flake off, or to effect the disintegration of the body. The speed is dependent upon the temperature and relative humidity of the air into which the object is introduced. Every salt has a critical relative humidity at which crystallization occurs, and the presence of more than one type of salt can affect their behavior traits.
To summarize - causitive conditions/agents:
a) Direct exposure to rain watering, salt spray/high RH.
b) Skin/sealing effect from applications of "overpaint"trapping soluble salts/moisture.
c) Influx of salts/moisture from limestone base sitting directly on the ground. It must be noted that the limestone was wet through at the time of our examination and that there was a mineral shadow left on the top surface that coincided with the urn's foot. It is probable that some water and salts migrated up through the limestone into the urn.
To address question #2 "What are your suggestions to stop this decay?"
Removing it from direct exposure, although this will likely only slow down the processes that are in progress.
a) Move it indoors, but as above, will likely not stop decay.
b) Remove to a low RH environment (below 50 % RH) - not too practical unless you have a climate controlled display case made.
c) Desalination (through poultice or soaking) if there are soluble salts, otherwise, any consolidation will probably make the problems worse in a few years.
d) Consolidate with an acrylic resin and display indoors.
Note: This question does not address aesthetics or any degree of filling in areas of loss with some inert fill material.
Another option would be to have a reproduction made and return that out into the elements. However, any ceramic (especially low fire, e.g. terra-cotta) will degrade sooner or later here in the islands. The more porous and the more exposure to the environment, the faster the decay will be.
To address question #3 "Can you estimate the age of the urn?"
Without taking a small sample of the body and having it analyzed by Thermoluminescence (TL) - which is costly, I will only say that it "appears" to not be a modern "fake". The resolution of TL can detect ages of ceramic objects of 50-100 yrs from those of 300-1000 yrs. Stylistically, they appear to be19th C. but I imagine similar is probably being made today.
And #4 "Do they look really old? New? Within 50 years?" Sorry, but am unsure.
In conclusion, leaving any porous ceramic item out in the elements here, is not advisable. With the urn, even if it is left in a covered area, you will likely begin to see efflorescence of salts on the surface, and/or further exfoliation and flaking of the clay (as evidenced by "terra-cotta" colored powder on the ground around the piece).
It had earlier been suggested that a coating of "polyurethane" be applied to the urn in an effort to stop the "decay". Over time, this unfortunately would make matters worse. All (practical) organic surface coatings have varying degrees of permeability to water vapor, so all coatings will have a net effect of allowing a higher concentration of entrained salts to build up just below the coating interface as fluctuations in the relative humidity occur. As stated elsewhere, this is largely if not solely the reason for the damage now evident.
It has not been my intention to present an overly complicated or ambiguous report for you, but rather to try and simplify many interacting and involved processes. This should be taken as a starting point for further discussion about what or how and by whom you wish to have the urn treated.
In an effort to get this information to you sooner than later, specific (involved) treatments and costs have been omitted here. When you have the opportunity to read this and view the photos, I would be happy to discuss different treatment strategies with you.
Clicking on any photo will link to larger size file. fig. 2 fig. 3 fig 4
Close ups of what appears to be an "overpaint" applied onto the surface. The majority of erosion and exfoliation occurs under these areas.
Below, areas of the repaired lip section, with detail close ups.
Detail views of side and under, top lip breaks fig. 10a fig. 10b fig.11a fig. 11b
* Special thanks to:
Stephen Koob, Conservator
Corning Museum of Glass