The effect of water boiling on annual ring width and porosity of cork.

Abstract

The effect of water boiling on cork was studied on 40 cork planks with a 9-year production cycle, i.e., containing 8 complete annual growth rings. The width of growth rings decreases from the 1st ring to the 8th ring (5.5 mm and 2.5 mm, respectively). All rings expanded in the radial direction with water boiling but differently: 12.8% for the 1st growth ring and increasing during the cycle to 26.9% for the 8th growth ring. On average, larger growth rings had smaller expansions. The initial porosity of cork was on average 8.2%, but it decreased with boiling to 4.0%, corresponding to an overall porosity decrease of 52%. The decrease depended on the size class of pores, being higher for larger pores: 55% decrease for pores >5 mm 2 and 45% for pores <2 mm 2 . The number of pores and their area decreased, especially of larger pores. With water boiling, there was an improvement of cork quality including corkboard flattening, a higher and more uniform thickness, and the decrease of the total area of pores and of visually important individual pores.