Conserving the hull of the "Mary Rose" was the most complicated and expensive task for the project. In 2002 a donation of from the Heritage Lottery Fund and equivalent monetary support from the Portsmouth City and Hampshire County Councils was needed to keep the work with conservation on schedule. During passive conservation, the ship structure could for practical reasons not be completely sealed, so instead it was regularly sprayed with filtered, recycled water that was kept at a temperature of 2 to 5 °C (35 to 41 °F) to keep it from drying out. Drying waterlogged wood that has been submerged for several centuries without appropriate conservation causes considerable shrinkage (20–50%) and leads to severe warping and cracking as water evaporates from the cellular structure of the wood. The substance polyethylene glycol (PEG) had been used before on archaeological wood, and was during the 1980s being used to conserve the "Vasa". After almost ten years of small-scale trials on timbers, an active three-phase conservation programme of the hull of the "Mary Rose" began in 1994. During the first phase, which lasted from 1994 to 2003, the wood was sprayed with low-molecular-weight PEG to replace the water in the cellular structure of the wood. From 2003 to 2010, a higher-molecular-weight PEG was used to strengthen the mechanical properties of the outer surface layers. The third phase consisted of a controlled air drying ending in 2016. Researchers are planning on using magnetic nanoparticles to remove iron in the ship's wood to reduce the production of harmful sulfuric acid that is causing deterioration.