Stone specialist Marcin Pikus was asked by All Hallows Guild to provide much needed conservation work to significant Medieval stone features in Bishop’s Garden, a part of the 59 acres of the grounds of Washington National Cathedral.

Marcin Pikus holds a Master’s Degree in stone conservation from Nicholas Copernicus University in Turin, Poland and has accumulated significant experience with stone in Europe.

All Hallows Guild was founded in 1916 to raise support and funds for the planting and features of the Bishop’s Garden and the Cathedral Close. Stone features in the garden include medieval sculpture from George Gray Barnard, whose collection formed the basis of the Cloisters in New York.

The stone conservation treatments were performed during October 2018 and items treated involved a group of five granite reliefs and one freestanding limestone baptismal font listed below:

  1. Crucifixion of Christ with Mary and John
  2. St. Catherine and other Figures
  3. Kneeling Figure A
  4. Four Figures
  5. Kneeling Figure B
  6. Carolingian Baptismal Font

CONDITION OF THE RELIEFS

The condition of all the reliefs was between fair to poor.  They presented similar deterioration patterns; however, the alterations found on the first relief (Crucifixion of Christ with Mary and John), located within the shady and humid microclimate of the Norman Court, were more extensive.  The green layer of biofilm covered the entire surface of the relief, while the remaining four (mounted on the east-facing retaining wall) were only partially affected by the microorganisms in form of black staining.  These reliefs also exhibited extensive accumulation of loose soiling on their surfaces.  The soiling was created by muddy water running down the stone wall from the ground surface above.

All reliefs exhibited serious degradation of the original surface. The details of the carvings had been reduced to rounded forms, as their original surfaces continued to gradually disintegrate due to factors related to the almost perpetual presence of water, changing thermal conditions and extensive microbiological growth.

CONDITION OF THE BAPTISMAL FONT

The font is located in the center of a little herb garden called “Hortulus” and is a three-segment stone object consisting of an octagonal bowl resting on an octagonal prism-shaped column with an octagonal base.  The bowl is carved from one large piece of a very dense limestone (colloquially called “marble” because of its dense structure), while the column and the base are carved from two separate blocks of true limestone.

The condition of the font was poor.  All three components exhibited a considerable degree of deterioration and were extensively covered by a thick layer of microorganisms (mostly algae, lichen and fungi).  The biofilm found on the bowl surface was dark grey, while the less intense staining found on the column and the base was green.  The bowl exhibited an extensive number of cracks and fissures.  A large loss was found on the upper part of the side facing east.  The loss was filled with a grey colored cementitious mortar installed in the past. The fill was partially deteriorated and did not match the adjacent areas visually.  The concave surface of the bowl was covered with cement-sand mortar and was partially deteriorated.  The surface was also dark-stained by microorganisms.

TREATMENT OF THE RELIEFS

The first step of the conservation treatment of the reliefs was a thorough cleaning procedure and removal of the biofilm from the stone surface.

The cleaning was followed by an application of a biocide (D/2 Biological Solution) and thorough rinsing with water.

The final step of the treatment was a consolidation of the weakened stone surface with Conservare H100 (PROSOCO).  This procedure was a combination of consolidation/water-repellent treatment.  The ethyl silicate treatment, modified with a silane water repellent, introduced new natural binding materials while protecting the treated surface from water-related deterioration.  The application strictly followed manufacturers’ recommendations.

Three consecutive saturation cycles (three applications in each cycle) were performed and the stone surface was covered with plastic to protect the stone surface from rain for one-week period.  The plastic was later removed by the Bishop’s Garden staff.

TREATMENT OF THE BAPTISMAL FONT

The limestone baptismal font conservation treatment began with a cleaning procedure. The stone surface was pre-soaked with D/2 Biological Solution for a period of 15 minutes before cleaning to disinfect the stone surface and to facilitate the cleaning process.

The cleaning was completed by an application of 8% aqueous solution of Calcium Hypochlorite and thorough rinsing.  The cleaning procedure resulted in complete removal of the biofilm and dramatically improved the appearance of stone.

The cleaning was followed by a series of injections aimed at structural consolidation of the cracked limestone surface.  Each of multiple cracks found on the bowl’s surface was filled with 20% acrylic dispersion (Rhoplex B60 A/ DOW).  The bonding agent was injected using a syringe with a needle, which allowed for precise injection of the emulsion into the crack.

The excess of the adhesive was removed with acetone and denatured alcohol.

The old and unsightly cementitious fill was removed with chisel and hammer.  The loss was filled with color matched marble restoration mortar Jahn M120.  The pigments used to modify the base color of the mortar were: yellow ochre, raw umber, transparent earth red and natural sienna.  The joints between the three elements were filled with color matched restoration mortar Jahn M70.

The concave surface of the bowl was coated with an acrylic/cementitious coating Thoroseal (THORO).  The original white color of the coating was modified with pigments to match the color of the adjacent limestone.

The final step of the treatment was the saturation of the stone surface with Conservare H100 (PROSOCO).  This procedure was a combination of consolidation/water-repellent treatment.  The ethyl silicate treatment, modified with a silane water repellent, introduced new natural binding materials while protecting the treated surface from water-related deterioration.  The application strictly followed manufacturers’ recommendations.  Three consecutive saturation cycles (three applications in each cycle) were performed and the stone surface was covered with plastic to protect the stone surface from rain for one-week period.  The plastic was later removed by the Bishop’s Garden staff.