Sheffield City Hall
Sheffield City Hall is a Grade II* listed building which dominates Barker's Pool, one of Sheffield's central squares. It is a neo-classical building with a giant portico. Built of Darley Dale sandstone, Sheffield City Hall opened in 1932 three years after the foundation stone was laid in 1929. Construction of City Hall was delayed 8 years because of the economic climate in the early 1920's. In its day the City Hall boasted one of the most modern air conditioning plants in the country and was frequently demonstrated as an example of the best possible type of modern ventilation.
During the Second World War a bomb exploded in Barker's Pool, damaging the pillars of the building. The scars of the explosion can still be seen to this day.
In 2005 Barker's Pool was renovated and relaid in Yorkshire stone. Two fountains were installed outside the City Hall to create a link with the historic use of the square as the town reservoir. These comprise of two square polished granite plinths.
Stone Hardness
The Mohs scale of mineral hardness is a qualitative scale that characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. Despite its simplicity and lack of precision, the Mohs scale is highly relevant for field geologists, who use the scale to roughly identify minerals using scratch kits. Different materials have different hardness, with a lower number on the Moh scale meaning a less hard material quartz (one of the main components of various types of rock due to its abundance) has a hardness of 7, while steel has a hardness of around 5, which can increase to 8 if the steel is hardened.
Another measure of hardness is the Vickers scale. The basic principle is to observe the questioned material's ability to resist deformation from a standard source. There are two types of deformation: elastic (where the material snaps back to it's original shape after the impact) and plastic (where an impact crater is left). The measure is given as kg/mm2, which means the number of kilograms per square millimetre it can withstand before fracturing, so is similar to pressure. Quartz can withstand 1200kg/mm2, a lot stronger than hardened steel at 900kg/mm2.
As you can see from the above measures of hardness, the hardness of various stones will vary depending on the composition of the stone.
Logging your Find
In order to log your find, please send a message to my profile answering the below questions. There is no information about the stone around City Hall, so you may need to do some research to answer these questions, the answers to which are freely available on the internet.
In addition to the below questions, optionally upload a picture of yourself or your GPS with the largest shrapnel damage you can find. You can log your find immediately, but please send the answers within 24 hours. I will respond to your email as soon as I can. Logs where incomplete answers have been given, or where an email has not been received, may be deleted.
- Go to Waypoint 1 (N53 22.836 W001 28.327).
- Find a large piece of shrapnel damage to the pillar. Approximately how large are the impact craters?
- Using the Moh and Vickers scales detailed above, what can you tell me about the relative hardness of the stone and the shrapnel that can explain why an impact crater was left, and how the stone absorbed the impact?
- Imagine the granite fountains were in place when the shrapnel hit. What difference would you expect in the size and depth of the shrapnel damage to granite rather than Darley Dale sandstone?
