In this century, there is a worldwide upsurge in the use of weathering steel for open-air sculptures. This guide is meant to present some sort of footing for conservators to employ when making considerations for possible treatments when taking care of weathered steel sculptures.
What is Weathering Steel?
Weathered steel, officially known by BlueScope Plate Supplies’ trade name XLERPLATE®, is a low-alloy structural steel that forms an adherent rust coating when it is subject to atmospheric changes.
What does ‘Weathering’ Mean?
Natural processes are responsible for the appearance of sculptures that are crafted using this structural steel. For that reason, XLERPLATE® is often referred to as ‘weathering steel’, as an analogy to the weathering of biological materials such as wood.
Weather conditions in Australia variate from day to day. One day it would be hailing while the next it could be torrential winds.
This ‘weathering’ characterisation is meant to remind you that the material has the ability to change its outer shell overtime when exposed to the environment. Conservators should always keep this in mind when they appreciate and care for works that are executed using this material.
What is Responsible for the Look of Weathered Steel?
Weathered steel becomes adherent only after the fine discrete crystallites of the initial layer of rust recrystallise into a fairly intact ‘patina’. For this protective rust patina to be formed there must be alternating cycles of wetting and drying – the wetting allows for rust generation whereas the drying cycle is necessary for it to recrystallise.
If the structural steel lacks ample drying time, the resultant continual rusting will cause the partly-crystallised outer rust layer to flake off. Therefore, there will be no formation of the protective patina.
That said, we can conclude that the appearance of weathering steel is solely dependent on the extent of formation of its patina – essentially, the degree to which re-crystallisation occurs.
The initial rust takes shape into fine discrete crystallites that are seemingly red and ones that are diffusely-reflecting – similar to those of hematite. As for the colossal re-crystallised layer, it develops into shiny-blue which closely resembles the blue-black look of the specular hematite.
Provided that different weathered steel sculptures are exposed to different levels wetting and drying cycles, they will all have variant degrees of re-crystallised oxide – and thus, different appearances.
4 Factors that Affect the Look of Weathering Steel Sculptures
Here are some of the factors that affect the appearance of weathered steel sculptures:
Environmental and Climatic Conditions
In most environments, nearly all surfaces of outdoor weathering steel sculptures are exposed to varying levels of wetting and drying cycles. Given that all these different environments have variant counts of re-crystallised oxide, the XLERPLATE® sculptures will surely have contrasting appearances.
Skyward surfaces of a sculpture will generally be bluer and glossier in their look because they are more exposed to the drying cycle. On the flipside, the ground-ward surfaces will be seemingly red and flatter as they see more of the wetting cycle through condensation and runoffs.
The runoff water from the skyward sculpture surfaces will end up creating lifelong streaks or other moulds of seemingly red oxide to the surfaces facing the ground. Similarly, the overall look of XLERPLATE® sculptures in wetter climates will be ostensibly red in appeal as compared to those exposed to drier climatic conditions.
Conservators might want to keep in mind that heavier weathered structural steel plates are often manufactured with low-alloy contents than the thin sheets.
Why? A plate that is of low-alloy content provides a rust coating that is less adherent, one that has a higher rate of corrosion and one that flakes off a whole lot more. Such a plate will only present a rust coating that is certainly younger and apparently red in its appeal.
Coatings and Graffiti
Other factors such as graffiti and paint coatings can also impinge on the look of weathering steel sculptures. All through the re-crystallisation process, rust coops up particulate matter on the sculpture’s surface.
In the case that this particulate matter has some colouring, the look of the forming rust will assume some of these aspects. For example, the rust on weathered structural steel structures in industrial atmospheres will more often than not be black because of the airborne dirt.
Chemical Cleaning Treatments
The chemical compounds, such as acids, that conservators use to clean weathering steel sculptures can convert the rust coating (hydrated iron oxide) on the surface to other iron compounds that may be of different looks.
3 Maintenance Practises on Weathering Steel Sculptures
Any discoloured portions on a weathered steel sculpture could be brought about by excessive corrosion or one, or more, of the factors highlighted above. Most Australian states have favourable climatic conditions for open-air sculptures.
So in most cases, maintenance efforts should not have any effect on the strength of the work. Here are a couple of ways conservators can use to care for weathering structural steel:
Coating and Reinforcement
Unseemly design pockets, or crevices, on weathering steel sculptures will be a trap for water trickling down from skyward surfaces. The constant presence of water will certainly spark excessive corrosion levels – noticeable through rust flakes and metal loss.
To avoid any further deterioration of a weathering structural steel sculpture, conservators should seal or coat all crevices and pockets. It is crucial to note that the use of a clear sealer, such as polyurethane varnish, will impede the routine weathering process.
Even though the seal might require nominal restorative treatment, it will certainly alter the sculpture’s look. In the case that there is evident loss of steel, conservators should consider reinforcement options.
Just as it is with hot-rolled carbon steel, newly manufactured weathering structural steel develops a mill-scale layer. Then again, the XLERPLATE® will often assume a dark purple colouring in contrast to the conventional slate blue colour.
Therefore, conservators can opt to nominally abrasive-blast clean the surfaces of sculptures if there are no intents to coat it. This allows for the consistent formation of the patina across all surfaces of the sculpture.
Conservators might want to keep in mind that the evenness of the patina layer only adds to the aesthetic value of a sculpture – it will have no noteworthy effect to the overall performance of the uncoated weathered steel.
Chemical Cleaning Compounds
Conservators can use chemical compounds to spark the formation of a patina layer on the surfaces of a weathering steel sculpture that has already undergone rust removal practices. Case in point – A conservator can dissolve a 5-10 percent solution of hydrochloric acid in water and use it to clean the surfaces to realise an appealing look.
Now that you, as a conservator, appreciate and care for weathering steel sculptures, why not get in touch with one of BlueScope Plate Supplies’ branches for a no-obligation real time chat?
Our dynamic and contemporary company services earn us the right to guide you in the treatment of open-air weathered steel sculptures. We understand that the weather variates and changes from day – to – day, so why worry about something thats so simple like steel. We have and distribute an array of steel plate products to choose from- TRU-SPEC® and XLERPLATE® .