MISCELLANEOUS TERRA COTTA LABORATORY NOTES*

The following points are discussed in this paper: (1) green scumming, (2) black ash as a substitute for barium carbonate, and (3) colemanite overglazes for polychrcnne.     

DISCOLORATION OF A TERRA COTTA GLAZE*

This paper deals with a peculiar discoloration of a typical terra cotta glaze. The method of solving the problem is outlined, and conclusions are reached which point out methods of prevention of similar defects.     

BETTER TERRA COTTA SLABS1

Terra cotta slab problems are discussed as to (1) refractoriness, (2) mechanical strength, (3) accurate dimensions, (4) temperature changes, (5) spalling, (6) thermal conductivity, and (7) abrasion. Mixes of two experimental bodies are given. The second consisting of 30 grog, 15 clay of low plasticity and 55 bond clays gave satisfactory results.     

CONSTRUCTION OF WATER-TIGHT BRICK MASONRY1

About 300 panels have been erected to determine the effect of some of the various factors of different brick, mortars, workmanship. and design in causing leaky brick walls. The brick varied in absorption from 0.6 to over 12% of their weight when placed flat side down in 1/8 inch water for ten minutes. The mortars included both lean and the richest mixes of lime and Portland cement in varying proportions and of a large number of masonry cements. Most of the panels one brick in thickness leaked in half an hour on being sprayed with a hose. A very few mortars were found that were quite tight. Those panels with an air space between the front and the backing allowed no water to pass clear through the wall. Many panels have been broken in flexure with very interesting and important results. Other panels have been built to test out interior ventilation of walls, and many other points have been studied.     

TERRA COTTA KILN SLAB INVESTIGATION1

Following several discussions as to the constitution of a good kiln slab for terra cotta manufacture, a coöperative test was made in which slabs of various compositions were manufactured by a producer of refractories, tested under actual service conditions, and physical measurements were determined by the Ceramic Department, Rutgers University. Series included the use of tight-firing medium refractory clays, open-firing refractory clays with grog, both vitreous and porous. Grogs were sized (definite proportions of size ranges constituting the different members). Conclusions were drawn as to effect of all these variables.     

POSSIBILITIES OF TERRA COTTA CASTING

Experimental method.—The casting properties, viscosity, specific gravity and drying behavior of three kaolins, two ball clays, one fire-clay and four representative commercial bodies, together with terra cotta clays, were studied. The clays and bodies were treated with varying amounts of salts and cast in the form of small ashlars. Conclusions.—Kaolins have an open structure but do not develop strength on drying. Bonding properties are furnished by ball clays which, however, tend to seal against the mold. Commercial terra cotta bodies tested do not possess good casting properties but can be modified through a study of the component clays. Mechanical difficulties attending the casting of terra cotta render the feasibility of the process doubtful at the present time.     

NOTES ON FIRING TERRA COTTA1

Some notes are given on the firing behavior of a 14-foot diameter muffle type terra cotta kiln, using coal as a fuel. The need of more data on periodic fired kilns in all branches of the ceramic field is stressed.     

TERRA COTTA IN ARCHITECTURAL DESIGN1

Emphasis is made of the use of terra cotta as an artistic and decorative material. Its successful use demands an architecturally educated talent and a ceramic chemist who appreciates texture, hue and the required colors. The tendency to employ terra cotta to bring out effects of stone masonry design is shown.     

SOME FUNDAMENTALS OF TERRA COTTA1

Terra cotta is not a definite material. Considerable difference of opinion exists as to suitable structure and composition. There are more differences between different makes of terra cotta than there are in opinion as to what it should be. Microscope study of thin sections of terra cotta is a valuable method of examination. The voids in terra cotta are of more than one kind and origin. The grog used in terra cotta is frequently unsuitable as to character and size. A fuller knowledge of fundamentals will probably much alter ideas as to suitable grog. A test for internal stress is described. Terra cotta was tested for osmotic action. The internal degradation of terra cotta is demonstrated by two different methods. A number of slides are presented being typical of certain characteristics.     

NOTES ON SHIVERING OF TERRA COTTA1

Six terra cotta clays are tested for shivering as a body trouble using a white mat glaze, at cone 3 starting. One non-shivering clay and one badly shivering clay are experimented with, to produce and overcome shivering. Causes of Shivering .—From this work and that of others which has been checked, uses are

• 1 presence of finely divided silica in the body, from

• (a) highly siliceous clays
• (b) finely ground grog of a siliceous nature

• 2 presence of soluble salts in the clay, grog, or tempering water
• 3 longer firing periods

Methods of Eliminating Shivering.

• 1 Use of a flux such as feldspar, felsite, or similar rock to overcome action of fine silica
• 2 Use of coarse grog or sand
• 3 Use of barium carbonate to overcame effects of salts

     

NOTES ON TERRA COTTA GLAZE CONSISTENCY1

Some of the difficulties encountered in applying glaze to terra cotta are discussed. The use of a hydrometer to test glazes is misleading as a hydrometer supplies gravity indication only in true solutions and glazes are not true solutions; furthermore the viscosity of a glaze may obscure the real specific gravity. It is suggested that in order to Control a glaze it should first be made to a desirable specific gravity and then the desired viscosity at that gravity be ascertained by experiment. A viscosimeter and specific gravity bottle are suggested for this purpose. The effect of change of alkalinity and of bacteria together with the control of glaze wastage are also discussed.