History and basic theory of Protective Glazing:

Protective secondary glazing as a method of conserving historic glass was first used in the 19thC. However it has been in common usage across Europe for about 50 years.

The technique has a simple aim. It had been recognized by conservation scientists that moisture, in the form of condensation, allied to airborne pollutants was the enemy of poorly durable high potash glass such as that used in the medieval period. This moisture was the cause of the pitting and corrosion, which can make colour and image more and more illegible as the glass body and the glass paint on its surface degrade. In the worst cases medieval glass had corroded through into holes, and was completely opaque.

The system devised aimed at separating the ancient glass from the damaging cycles of condensation by setting the ancient glass out of its' original position, slightly inside the building.

In earlier systems, new external glazing scheme was fitted in the original position of the stained glass.The ancient glass in its new position was ventilated at the top and bottom of each light to allow the warmer internal air to circulate between the two layers. In an ideal situation a 'flue effect' is arrived at to promote the airflow. The warmer internal air in the interspace between also acts as a thermal buffer to protect ancient glass from cycles of condensation. The glass is also protected from the ravages of the weather.

Philosophical debate concerning protective glazing.

There have been major disagreements in the U.K. between conservators and architects regarding the aesthetic impact of protective glazing. Many architects properly consider the conservation needs of the building as a whole, and do not wish to view the glass and its conservation needs in isolation, they also object to stained glass being divorced from its architectural context and displayed as they see it in 'museum conditions.'

It is interesting that the UK is the exception in this; European conservation architects seem to have embraced the concept of isothermal glazing. However we are certainly not apologists for the UK conservation situation and its routine of healthy debate.

The Holy Well Glass position is that we should engage in discussion about the undoubted aesthetic impact of protective glazing. However as we seek to engage and collaborate with specialist architects and other conservation colleagues, with the aim of producing ever more discreet solutions, we do expect an open minded discourse to develop. If a project is carefully evaluated and researched and the need for protective glazing clearly demonstrated, we trust that conservation architects will engage in the design process and in turn advocate the use of isothermal glazing.

When is protective glazing appropriate and what are the key considerations?

There is an unfortunate tendency towards employing the technique as a panacea for all stained glass with conservation needs. Some conservators believe that stained glass of any period which is failing in one of its primary purposes; simply keeping out the weather, should not be removed and re leaded in the traditional manner but fitted with protective glazing.

The logical conclusion of this is that eventually every church window in the land will be protectively glazed. Our unequivocal view is that this technique should be employed solely for ancient glass and 19th Century glass exhibiting problems with the chemical deterioration of the glass, or loss of painted detail.

We are against the blanket adoption of isothermal glazing; our reason for this is very simple and stark. If we allow this trend to gain momentum, the craft traditions of a thousand years will be placed under threat; we run the real risk that the skills of the glazier and glass painter will become irrelevant and skills will be irrecovably lost.

It is essential that we do not consider stained glass in isolation. In our view the masonry and supporting ironwork are very important too - we would certainly not for instance promote the removal and re-siting of ancient ferramenta as this is indefensible in our view; contrary to all conservation principles and guidelines.

We are also firmly of the opinion that attempts at producing a 'one size fits all' design, whereby an identical system is employed for all situations is not acceptable.Thus we engage and collaborate  with clients, conservation architects, and grant aiding bodies in designing greatly improved systems which are as discreet as possible and tailored to each church or building.

At  present we are refining systems where the ancient glass is retained in its original position and architectural context. a new protective external glazing layer is introduced, and the airflow between the new glazing and the ancient glass is contrived by building vents into the stained glass panels, this approach was pioneered at the renowned conservation studio at Cologne cathedral.

However we will design systems where the stained glass is moved inside the building where the complexity of stonework or data stemming from environmental monitoring suggest that this is the appropriate solution.

Partial protective glazing systems:

Over the last 15 years we have pioneered partial isothermal systems, whereby small sections of historic glass are set on to discreet bronze frames set inside the building. This can be very effective for instance where heraldic devices are set on to plain quarry glazing. Notable examples are at St. Albans cathedral, carried out in 2007 and at Cotehele House for the National Trust, which is ongoing.

This enables the principle of minimum intervention  to be closely adhered to.

Extensive experience in the design and installation of protective glazing:

Holy Well Glass have installed isothermal glazing systems at Salisbury Cathedral, St. Cross Church, Winchester, and many parish churche and great houses. We are in the process of installing isothermal glazing to the Great East Window at Wells cathedral- the celebrated 14th Century Tree of Jesse.

We have recently designed installations for Gloucester and Winchester cathedrals. 

Examples are illustrated below.

We are leaders in research and development for protective glazing for historic glass; our commitment to  refining the design of protective glazing systems is ongoing.

Through close collaboration with leading specialist in monitoring of historic building environment Tobit Curteis Associates,all of our installations are scientifically monitored to ensure that they work.

All installations are carefully designed to allow easy maintenance and cleaning without incurring major expense.

Please contact us should you require us to design such a system specifically for the building under your custodianship.