Dryice blasting

Another milestone in conservation of Tallinn Cog was just reached, when we finally saw the outer surface of the hull in its uncovered beauty, without the external supporting structures installed right after unearthing the wreck back in 2015. Furthermore, the so distorting orange corrosion stains were just stripped away as well.

Wow, what a sight!

Outer surface of the hull before dryice blasting. Note the orange corrosion deposits, soon to be removed. Photo: Eero Ehanti, FOG

As the readers of this blog know, we’ve been cleaning the inside surfaces of the hull manually, very slowly and very laboriously. This careful method was necessary because there were lots of ceramic-, textile-, leather-, bone- and whatnot fragments to be found within and in between the planks. Hence the very systematic excavation-kind of method described earlier, after which the beauty of the 13th century craftsmanship and enormity of the planks used can be truly admired. The outer surface of the hull, however, has been pretty much untouched so far due to the supporting structure covering it almost completely. It’s also a whole different story with mostly vertical surfaces. Another approach was needed here.

When the SCR Company started to straighten the hull, as described in the previous post, it was possible to take off some of the supporting structure. For the first time we could properly see the hull shape and measure the planks and identify where the joins are. Three first planks from the keel are carvel build in the middle and clinker at the bow and stern. From fourth plank up all the planks are clinker build. Clinker is a method where the plank edges overlap with each other. Overlapping planks are then fastened to each other and then to the frames. Lowest planks are massive; over 500 mm width and 60 to 70 mm thick. Average length of a planks is around 8 metres. Planks have been joined together with scarf joints, which are over 500 mm long.

the beauty of the huge planks after dryice blasting. Photo: Heikki Häyhä, FOG

But what to do with all those ugly orange iron corrosion deposits? Another round or two of manual work cleaning? After considering our options, it seemed impossible to carry out cleaning manually in a given timeframe. Also working conditions under the ship were not very convenient. How about dryice blasting? After deliberations about whether or not the method used f.i. for cleaning aluminium airplanes would be gentle enough, we decided to look into it. Surely it all sounds promising, not least because it’s a dry method. When -78 degrees of centigrade ice hits surface to be cleaned, it sublimates. So we wouldn’t be adding any moisture to the wood, which has been air-drying nicely for such a long time now. Air pressure shouldn’t be a problem either, as it can be easily controlled, and working distance is of course adjustable as well. So why not!

The method is similar to other forms of media blasting such as sand blasting, nut shell blasting or soda blasting. The difference to other blasting medias is that dryice blasting is nonabrasive, non-conductive, nonflammable and non-toxic. One great benefit of dryice blasting is that it does not create secondary waste or chemical residues as dry ice sublimates, or converts back to a gaseous state, when it hits the surface to be cleaned.

So we contacted a company called DryIce Finland and carried out a test with a loose planks from the hull. Result of the test were so promising that we decided to carry on with dryice blasting.

A date was set and DryIce Finland came to Tallinn with two blasting units and two operators. It took about five hours to clean the whole hull from outside. Parts of inner surfaces were give a finishing touch as well. Blasting was closely monitored and evaluated as the work proceeded. The result? Again, what a sight! We are indeed very happy with the result.