A 2006 study by Zanella, Gurioli, Pareschi, and Lanza used the magnetic characteristics of over 200 samples of lithic, roof-tile, and plaster fragments collected from pyroclastic deposits in and around Pompeii to estimate the equilibrium temperatures of the deposits.
The deposits were placed by pyroclastic density currents (PDCs) resulting from the collapses of the Plinian column.
The authors argue that fragments over 2–5 cm (0.8–2 in) were not in the current long enough to acquire its temperature, which would have been much higher, and therefore they distinguish between the depositional temperatures, which they estimated, and the emplacement temperatures, which in some cases based on the cooling characteristics of some types and fragment sizes of rocks they believed they also could estimate.
Final figures are considered to be those of the rocks in the current just before deposition.

All crystal rock contains some iron or iron compounds, rendering it ferromagnetic, as do Roman roof tiles and plaster.
These materials may acquire a residual field from a number of sources.
When individual molecules, which are magnetic dipoles, are held in alignment by being bound in a crystalline structure, the small fields reinforce each other to form the rock's residual field.
Heating the material adds internal energy to it.
At the Curie temperature, the vibration of the molecules is sufficient to disrupt the alignment; the material loses its residual magnetism and assumes whatever magnetic field might be applied to it only for the duration of the application.
The authors term this phenomenon unblocking.
Residual magnetism is considered to "block out" non-residual fields.

A rock is a mixture of minerals, each with its own Curie temperature; the authors therefore looked for a spectrum of temperatures rather than a single temperature.
In the ideal sample, the PDC did not raise the temperature of the fragment beyond the highest blocking temperature.
Some constituent material retained the magnetism imposed by the Earth's field when the item was formed.
The temperature was raised above the lowest blocking temperature and therefore some minerals on recooling acquired the magnetism of the Earth as it was in 79 AD.
The overall field of the sample was the vector sum of the fields of the high-blocking material and the low-blocking material.

This type of sample made possible estimation of the low unblocking temperature.
Using special equipment that measured field direction and strength at various temperatures, the experimenters raised the temperature of the sample in increments of 40 °C (70 °F) from 100 °C (210 °F) until it reached the low unblocking temperature.
Deprived of one of its components, the overall field changed direction.
A plot of direction at each increment identified the increment at which the sample's resultant magnetism had formed.
That was considered to be the equilibrium temperature of the deposit.
Considering the data for all the deposits of the surge arrived at a surge deposit estimate.
The authors discovered that the city, Pompeii, was a relatively cool spot within a much hotter field, which they attributed to interaction of the surge with the "fabric" of the city.

The investigators reconstruct the sequence of volcanic events as follows:
On the first day of the eruption a fall of white pumice containing clastic fragments of up to 3 centimetres (1 in) fell for several hours.
It heated the roof tiles to 120–140 °C (250–280 °F).
This period would have been the last opportunity to escape.
Subsequently, a second column deposited a grey pumice with clastics up to 10 cm (4 in), temperature unsampled, but presumed to be higher, for 18 hours.
These two falls were the Plinian phase.
The collapse of the edges of these clouds generated the first dilute PDCs, which must have been devastating to Herculaneum, but did not enter Pompeii.

Early in the morning of the second day the grey cloud began to collapse to a greater degree.
Two major surges struck and destroyed Pompeii.
Herculaneum and all its population no longer existed.
The emplacement
A 2006 study by Zanella, Gurioli, Pareschi, and Lanza used the magnetic characteristics of over 200 samples of lithic, roof-tile, and plaster fragments collected from pyroclastic deposits in and around Pompeii to estimate the equilibrium temperatures of the deposits. The deposits were placed by pyroclastic density currents (PDCs) resulting from the collapses of the Plinian column. The authors argue that fragments over 2–5 cm (0.8–2 in) were not in the current long enough to acquire its temperature, which would have been much higher, and therefore they distinguish between the depositional temperatures, which they estimated, and the emplacement temperatures, which in some cases based on the cooling characteristics of some types and fragment sizes of rocks they believed they also could estimate. Final figures are considered to be those of the rocks in the current just before deposition.
All crystal rock contains some iron or iron compounds, rendering it ferromagnetic, as do Roman roof tiles and plaster. These materials may acquire a residual field from a number of sources. When individual molecules, which are magnetic dipoles, are held in alignment by being bound in a crystalline structure, the small fields reinforce each other to form the rock's residual field. Heating the material adds internal energy to it. At the Curie temperature, the vibration of the molecules is sufficient to disrupt the alignment; the material loses its residual magnetism and assumes whatever magnetic field might be applied to it only for the duration of the application. The authors term this phenomenon unblocking. Residual magnetism is considered to "block out" non-residual fields.
A rock is a mixture of minerals, each with its own Curie temperature; the authors therefore looked for a spectrum of temperatures rather than a single temperature. In the ideal sample, the PDC did not raise the temperature of the fragment beyond the highest blocking temperature. Some constituent material retained the magnetism imposed by the Earth's field when the item was formed. The temperature was raised above the lowest blocking temperature and therefore some minerals on recooling acquired the magnetism of the Earth as it was in 79 AD. The overall field of the sample was the vector sum of the fields of the high-blocking material and the low-blocking material.
This type of sample made possible estimation of the low unblocking temperature. Using special equipment that measured field direction and strength at various temperatures, the experimenters raised the temperature of the sample in increments of 40 °C (70 °F) from 100 °C (210 °F) until it reached the low unblocking temperature. Deprived of one of its components, the overall field changed direction. A plot of direction at each increment identified the increment at which the sample's resultant magnetism had formed. That was considered to be the equilibrium temperature of the deposit. Considering the data for all the deposits of the surge arrived at a surge deposit estimate. The authors discovered that the city, Pompeii, was a relatively cool spot within a much hotter field, which they attributed to interaction of the surge with the "fabric" of the city.
The investigators reconstruct the sequence of volcanic events as follows:
On the first day of the eruption a fall of white pumice containing clastic fragments of up to 3 centimetres (1 in) fell for several hours. It heated the roof tiles to 120–140 °C (250–280 °F). This period would have been the last opportunity to escape. Subsequently, a second column deposited a grey pumice with clastics up to 10 cm (4 in), temperature unsampled, but presumed to be higher, for 18 hours. These two falls were the Plinian phase. The collapse of the edges of these clouds generated the first dilute PDCs, which must have been devastating to Herculaneum, but did not enter Pompeii.
Early in the morning of the second day the grey cloud began to collapse to a greater degree. Two major surges struck and destroyed Pompeii. Herculaneum and all its population no longer existed. The emplacement 
Eruption of Mount Vesuvius in 79 AD