Diagnostics and preparation of restoration works of a 36 m high stone column holding a giant bronze statue

One of the most visited historical sites of Budapest is the Heroes’ Square, where a 36 m high Corinthian-style stone column forms part of the Millennium Monument. On the top of the column, a bronze statue depicting Archangel Gabriel is visible. The monument was inaugurated in 1901. The stone column consists of ring-shaped limestone segments (drums). In the conical shape, the drums have a diameter of 2.25  at the lower part of the column, which reduces to 1.9 m at the top. Each drum is 0.5 m high. The condition of the stone column has aggravated in the past 120 years and the current research provides an overview of the actual condition of the stone elements and gives some hints on the preservation of the bronze statue. Tests included the on-site identification of major lithotypes, and testing surface strength and weathering grade using a Schmidt hammer. The moisture distribution was detected using a portable moisture meter. Small samples were collected for laboratory analyses and oriented samples were taken representing the north, east, south and west directions, with different exposure to sun and precipitation, representing different micro-climates. Textural analyses of samples were made by using optical microscopy and SEM-BSE, while mineralogical composition and elemental analyses were performed using XRF and thermogravimetric analyses. The prevailing lithotype is Pleistocene travertine with some textural varieties. The most common weathering forms are black crusts, dissolution features, and green staining of the stone surface from dissolved bronze. The composition of weathering crusts differs from the orientation, namely wind and rain-exposed parts have a different composition than the sheltered ones. The stability of the column was also analyzed. During structural analyses, a minor amount of cracks were also recognized. Loss of renders and dissolution of the stone surface can also cause problems. The study provides an example of integrated research of material properties and structural stability of historic stone structures.