In a widely cited paper, Johannes Hertz raises a number of arguments against a pre-Colonial origin for the famous Newport, Rhode Island Stone Tower. Hertz insists that it was modeled after the 17th century Chesterton Mill in Warwickshire, England, and points out that a survey by Hugh Hencken and William S. Godfrey found indisputably colonial artifacts at the bottom of a trench that surrounds the foundations.
According to HJ, their tests indicate that the Tower was built not earlier than AD, and most likely in the range Architect Suzanne Carlson, writing already in in response to the Danish original of Hertz's article, persuasively refutes Hertz's architectural and historical objections: Carlson argues that Chesterton Mill was in fact built as an observatory, and only much later converted to use as a mill.
She points out that the trench discovered during the survey makes sense as part of a colonial repair of a pre-existing tower for use as a windmill, after an earlier mill blew down in Furthermore, this trench does not work as part of the original construction, because it lacks any evidence of the presence of the staging that would have been necessary to have supported the arches.
Instead, its backfill contains thousands of mortar fragments, as would be expected if it were opened as part of a repair operation. However, Carlson admits that she, as an architect, does not understand the highly technical carbon dating of the mortar. I have had a little chemical training as an undergraduate at Caltech , and some prior familiarity with dendrocalibration, which is an important complication in the HJ paper.
Perhaps they or someone else will be able to correct me, but my reading of their paper is that although the C results are certainly consistent with a 17th century colonial origin for the tower, they by no means conclusively rule out a pre-Columbian origin.
To make lime mortar as was used in the Newport Tower, limestone mostly calcium carbonate is roasted to make calcium oxide, or quicklime. This is first combined with some water to make calcium hydroxide, or slake lime.
At the time of construction, additional water and sand are added to the slake lime, allowing it to absorb carbon dioxide from the air and to set into crystalline calcium carbonate. Some of the atmospheric carbon dioxide would contain radioactive C rather than inactive C or less common C, much as photosynthesis captures C from carbon dioxide in the atmosphere.
As originally devised circa , radiocarbon dating was based on the assumption that the proportion of C in the atmosphere has been constant over time, so that the amount of C left in a sample would fall exponentially with its age.
However, C dating of tree rings of known age subsequently demonstrated that this is not strictly true. Standard "dendrocalibration" curves have been constructed to compensate for this variation in atmospheric radiocarbon e. Stuiver and Kra, , and these curves are used by HJ. HJ admit that there are two factors that potentially could make the measured age of the mortar appear younger than the true age of the construction.
The first of these is slow absorption of atmospheric carbon dioxide. The surface of the mortar sets up quickly - in as little as a few hours.
The interior portions of the mortar will ordinarily set up eventually, but this requires carbon dioxide to diffuse through pores in the mortar, either in gaseous form, or in solution in the water that has been added to the slake lime. Depending on how easy it is for the gas to find such pores and work its way through them, this could take a considerable time.
Because of this slow absorption, the estimated date will not reflect the actual date of construction, but some weighted average of later dates, even if at the time of testing the reaction appears to have been complete.
Surprisingly, HJ do not report having even tested the Newport Tower samples for residual alkalinity. The second problem, which they just mention and then drop, is recrystallization of the carbonate.
It is well known that the calcium carbonate in bone is not very reliable for C dating, because the original carbonate ions may exchange with carbonate in the groundwater that might be either too old - if it represent dissolved limestone - or too young - if it contains atmospheric carbon dioxide dissolved in rainwater.
For this reason, the preferred method of dating bone is to isolate the bone collagen, which contains carbon, but in a more stable form. Similarly, lime mortar that is exposed to rain on a regular basis may contain carbonate that dates not from when the mortar first set up, but from much later rainstorms that may have drenched the structure. A third factor that might make their tests give too young a date, mentioned by Hertz but not by HJ, is the possibility of colonial repairs or "tuck-pointing" to strengthen a pre-existing structure for conversion to a windmill.
In this case, the surface mortar might be colonial, while only the deeper mortar, well inside the joints, would reflect the true date. According to Hertz p. Nevertheless, some of the samples HJ took were in fact "prized out as whole pieces of mortar, and marked as surface samples. HJ themselves make no mention that the other samples were taken with a care to avoid repairs.
The primary potential source of bias in the opposite direction, according to HJ, is unburnt limestone particles that may remain in the quicklime after roasting. These will remain as calcium carbonate in the mortar, but will contain essentially no C, and make the sample appear older than it really is.
These particles are harder than the mortar, and react more slowly with acid. In order to minimize their effect, HJ take the trouble to separate the carbon dioxide that is released from the mortar into two fractions. They argue that the first fraction should contain little if any of the fossil carbonate, and indeed the second fractions of the drilled samples give dates that are older, by as much as years.
For this reason they discard the second fraction dates, except for the surface samples, for which mechanical separation of limestone particles was feasible. As a control to see how accurate their method is, HJ also date a mortar sample taken from the nearby Wanton-Lyman-Hazard WLH House, known to have been built between and The date they obtain is consistent with its known date of construction, and so they conclude that their date on the Tower must be accurate as well. Although testing this house was a sensible and useful control, there are three problems with its interpretation.
The first problem with the WLH House control is that due to the peculiarities of the dendrocalibration curve over the past few centuries, it tells us almost nothing about the rate of initial absorption of carbon dioxide into mortar. Because of coal burning during the Industrial Revolution, tree rings known to date from approximately to contain very similar amounts of carbon today, and hence dates in this period are very difficult to differentiate.
Thus, although the WLH House gives a point estimate date that can be dendrocalibrated to AD, the same raw C content also dendrocalibrates, using HJ's Figure 2, to approximately , , , or even ! In other words, the WLH House contains mortar whose carbonate could have set at any time between its known date of construction and the midth century. If the method has a bias due to slow absorption, this control therefore tells us nothing about it.
The second problem with this control is that the sample obtained from it was apparently from its interior in the basement , where it would have been protected from the elements, and thus not prone to carbonate substitution from rainwater.
The Tower, on the other hand, would have been open to the elements for centuries before its colonial conversion to a windmill, if indeed it is medieval, and in any event has been again exposed to the weather in recent times, since at least , according to Hertz.
The third problem with the WLH House as a control is that it only tells us that the mortar method correctly dates late 17th century structures. In order to demonstrate that their test results rule out a pre-Columbian Norse date for the Newport Tower, they should have also provided one or more controls known to have approximately the alleged pre-Columbian date of the Tower, and which were exposed to the same sort of weather the Tower would have received. They therefore are not true controls.
And even if their true ages were known, the samples were taken from the interior, protected portions of these churches, which apparently have been continuously roofed since their construction. See HJ, Figures 3 and 4.
These tests therefore tell us nothing about the rate of substitution from rainwater that may have occurred in the case of the Newport Tower. When we scrutinize HJ's Table 1, we find some further problems with their dates. However, they do not explain what is present, if not calcium carbonate. If the difference is primarily unreacted calcium hydroxide, there is a serious slow reaction problem that potentially affects all the dates.
On the other hand, if the other material is primarily inert silica sand, there is no particular indication that the remaining calcium carbonate is in any way contaminated, and these samples should be no worse than any of the others. Since Accelerator Mass Spectrometry AMS methods are being used to measure the amount of C, valid results can often be obtained with even very small amounts of carbon. Small sample size might result in a large standard error, and therefore could be a valid criterion for forgoing the expense of a test, but is not per se a valid criterion for rejecting test results once they have been performed, so long as the standard error is not unusually large as a result.
Thus, two samples contained 2. However, sample 8 from the fireplace was tested despite containing only 5. It is very significant that the preferred first fraction of carbon extracted from the inappropriately excluded sample 8 gives a negative uncalibrated C date of BP, or in other words, AD!
Because atmospheric testing of nuclear weapons since has made recent decades appear to be far in the future before calibration, this does not literally mean that the mortar tested in had atmospheric carbon from the 21st century. However, it does indicate, after calibration, that the carbon was from some date after , long after the Tower is known to have been built.
HJ make no comment on this impossible date for the construction, but instead merely drop it from consideration on the inappropriate criterion of the low carbonate concentration per se.
In fact, sample 8 appears to exhibit a more severe case of substitution bias than I would have imagined possible, despite Hertz's assurances , p. This sample was tested twice, in two preparations of the same mortar sample. The first sample, 2. However, the later carbon dioxide consistently gives older dates for the drilled samples, due to slower dissolution of particles of unburnt limestone that remained in the lime, whence HJ's preference for the first fraction, so that if anything this difference should make the first sample appear younger than the second.
HJ make no comment on this inconsistency, but instead treat 2. A third anomaly in their data comes from the carbonate concentration they report for the WLH House sample. If this is true, there is something wrong with the chemical model being used that ought to be investigated. One additional factor that may affect the results, is the possibility that the lime mortar reaction may not fractionate C at the same rate that photosynthesis does.
Isotopic fractionation occurs because C atoms are heavier than ordinary C atoms, and therefore carbon dioxide molecules containing C move about more slowly at any given temperature.
This means that calcium hydroxide will have a higher probability of reacting with molecules containing C than with those containing C, even if these were present in the same proportions.
Photosynthesis, which the dendrocalibration curve is based on, will similarly fractionate C differently than C, but possibly at a different rate than the mortar reaction. Furthermore, molecules containing C will diffuse through the pores in the mortar more slowly, and hence will reach its interior to react in reduced proportions. In the end HJ apparently use five dates - two dates on the questionable surface sample from the flue, the contradictory dates from the two preparations of sample 2 from pillar 7 first fraction only , and the first fraction of sample 12, from pillar 6 - to date the Tower.
The point estimate dendrocalibrates to AD, but also to approximately , as well as , by their Figure 2. Although HJ conclude from their results that the Newport Tower could not have been built before , I regard this as inconclusive evidence against an earlier date for construction, for several reasons: Two of the dates used were from a surface sample that may have represented a colonial or even later repair to an earlier structure.
The inappropriately excluded post date on one of the samples tested demonstrates that rainwater substitution is an important factor. The Wanton-Lyman-Hazard House does nothing to demonstrate that the slow reaction and substitution biases are not a problem, because of the flatness of the dendrocalibration curve since AD on the one hand, and the fact that its sample was not exposed to the weather on the other hand.
The two Finnish churches do little to verify the Tower date, since the true dates of these churches are unknown, and since the samples were taken from the interior of the churches, where they were not exposed to the weather and potential carbonate substitution. Indeed, the fact that a few of the samples taken from them were still alkaline indicates that slow reaction may be a serious problem of mortar dating in general.
There are several inconsistencies in the results and unanswered questions that remain to be addressed. To be sure, none of these considerations proves that the Newport Tower is any older than I am merely returning a provisional "Scotch verdict" of "not proven colonial. Redwood Library and Athenaeum.
Postscript Since the above was written, the following very pertinent papers and websites have come to my attention: Theoretical Considerations," Newport History vol. Written and maintained by J. Huston McCulloch Send comments to: This is apparently either a bug in Netscape, or a bug in Windows' compatability with Netscape.