Preservation projects are invariably a balancing act between how much the preservation project will cost, what quality can be achieved, how long the project will take and what volumes of material can be preserved within that time. These four key factors of cost, quality, volume and time, are our four horsemen of preservation.

Making trade-offs between these factors is an engineering approach to preservation and aims to find the best compromise given the circumstances. This is sometimes referred to as the ‘cost of quality’ approach. In this way, the usual trade-offs between 'better', 'faster' and 'cheaper' can be explored.