The Process

Using a modern day manufacturing process, the Unilit range of natural hydraulic lime products replicate the strength, porosity, adhesion and flexibility of historic mortars.

Telling are credited with the reintroduction of hydraulic lime to the UK in the early 1990′s and lead the field in quality, range and performance in the use of natural hydraulic lime. Our knowledge and experience is without equal.

Developments in cement technology for volume construction in the 19th and early 20th century, together with the perceived advantage of increased mechanical strength, served to eliminate the need for the traditional skills of craftsmen to create a lime mortar. This has led to a distortion of the concept of mortar to such an extent that today a “traditional mortar” is defined as a blend of cement/sand/lime whereas it is little more than a modern process adopted to satisfy the demands of mass building.

From the beginning of this century until the mid 1970′s, the benefits of lime in conservation works were widely ignored with the result that many ancient structures were damaged by the application of incompatible mortars of cement and epoxy.

The advantages of lime conservation projects are now acknowledged, but due to its 50/60 year absence from production for construction use, the knowledge regarding performance of Hydraulic Lime is not fully understood. Many people consider Hydraulic Lime as being dense, strong and cementious, their source of information generally referring to the later 18th and 19th century Roman Cements used throughout the UK. These claims are generally true due to the production methods used at the time. Unilit products however, are Natural Hydraulic Limes – a mortar wholly distinct in its performance characteristics from those produced in the 18th and 19th century. Unilit is produced by the selection of a particular limestone which when burnt at a specific temperature, produces a mortar with optimum porosity, an initial set, and slow carbonation, these being the prerequisites for a compatible and sympathetic restoration mortar. The strength of the mortar is controlled by the ratio and sizing of sand to the lime mix and not by its hydraulicity i.e. feeble, moderate and eminent since these relate to the timing of the initial chemical set and not as is commonly believed, mortar strength.

Unilit Hydraulic Limes replicate the production of historic mortar which due to the local aggregates available and the imprecise way that they were burnt would have produced a hydraulic reaction of variable consistency. This is the complete opposite of modern day pure limes since these are the result of selective classification and burning of limestone.

This leads to the production of 99.9% pure lime to which pozzolanic material must then be added, sometimes even hydraulic lime to create a set. Pure lime is a material intended for chemical and agricultural usage, which was beyond the production capabilities of medieval craftsmen.Four of the most significant advantages of natural hydraulic lime over fat/putty lime are:

1. Its initial set, allows the conservator to make unimpeded progress throughout the year, especially during the winter months always provided that protection is provided for the first seventy two hours of curing rather than for several weeks.

2. The quality of manufacture ensures that even as a result of exposure to high levels of moisture extraction early in its set the natural hydraulic lime will not suffer from the unsightly contamination of the face of the mortar by free lime deposits (leeching).

3. Natural hydraulic lime tolerates the transfer of damp and salts without deterioration even if this occurs during the very early days of carbonation.

4. Enables work to be completed quicker, saving time and money.

The perceived difficulty associated with working with lime is removed by the introduction of Unilit Lime products. These have been developed for the restoration market by the considered evaluation of a modern process to produce traditional restoration products that do not require craftsmen’s knowledge for successful application.