GLAZE

GLAZING

We usually glaze bisque ware. Stoneware or porcelain clay bodies are normally bisque­fired between cones 010 and 06. At this stage the bisque is hard enough to be handled easily yet sufficiently porous to absorb glaze readily. It is possible to glaze ware in its raw state, but the glaze firing schedule that is used in the ceramics lab is inappropriate for green‑glazed ware. In other words, in this Studio: ALWAYS bisque fire before a glaze fire.

Before glazing, the ware should be clean and free of dust. The glaze must be completely removed from the bottom of the pot and about 1/4 inch up the side as soon as it is dry enough to handle. Excess glaze runs, so it is not advisable to allow a heavy layer of glaze to remain near the foot rim. Poorly glazed work will not be fired.

METHODS OF APPLICATION

Brushing: best suited for low‑temperature glazes, which are formulated so that brushing is easier and yields a more even coat. For high temperature glazes, it is difficult to apply an even coat using this technique, so that there is always danger of a streaky application.

Dipping: one of the best methods for achieving an even glaze thickness. The bisqued object is plunged into a vat of glaze, withdrawn almost immediately and shaken to remove the excess glaze. Finger marks may be touched up with a brushful or fingerful of glaze.

Pouring: requires less glaze than dipping and may be used with a great variety of forms. Most vessels are poured with glaze to coat the inside first. Almost immediately after pouring in, the glaze is poured out. Rotate the piece as you pour out, to evenly coat the inside. With preparation to capture the glaze, the outside of a piece can also be poured.

Spraying: permits subtle variations of color, and control over glaze thickness and coverage. Uses more glaze than dipping or pouring.

Underglazes and Slips:

Slips: must be applied to leather hard clay or slightly wetter. If applied too late, they will peel off. Slips can be fired to high‑fire and will retain their color.

Underglazes: Mayco underglazes are available in the Throwing Room. They are most successful applied to greenware. These colors are primarily low‑fire products and are covered with a clear glaze after the bisque firing.

GLAZE ROOM (#191)

Only currently enrolled ceramics students allowed in the glaze room at any time.

Please report any unauthorized people immediately,

Glaze materials are purchased with funds from your lab fee and are very expensive.

We need your help to maintain security.

SAFETY PRECAUTIONS:

1. Do not eat or drink in this room.

2. Wear an OSHA‑approved mask when spraying ceramic materials or creating clay dust.

3. Thoroughly clean up after use.

4. Label all containers.

GENERAL PROCEDURES ‑ WET GLAZING

1. Clean counters thoroughly before and after you glaze your work.

2. Do not leave small containers of glaze. Return all glazes to the large buckets. Wash the containers and return them to the storage shelves.

3. Return lids and stir sticks to glaze buckets immediately after use to avoid contamination.

4. Clean all equipment and containers after use.

5. Clean sink area after use.

GENERAL PROCEDURES ‑ DRY MIXING

1. Fully label all personal glazes and mixtures with your name, class, semester, type or name of glaze and its firing temperature.

2. Completely clean counter and scales after use. Vacuum floor if necessary.

3. Never remove any oxide or glaze materials container from the room.

4. Do not open bags without permission if contents cannot be put into bins immediately. Leave message for lab assistant if containers need filling.

5. Make sure scale pans are wiped clean and scales are set back to zero.

SPRAY BOOTH

Safety Precautions:

1. Always wear an OSHA‑approved protective mask when spraying ceramic materials or creating glaze or clay dust.

2. Spray or create clay dust only in the booth, making sure that the exhaust fan is turned on.

General Procedures:

1. Hook up spray gun hose to the compressor.

2. Spray your piece, turning the piece continuously.

3. After use, clean thoroughly.

SPRAY GUN

Safety Precautions:

Follow spray booth instructions.

General Procedures:

1. Plug hose into compressor.

2. Make sure siphon tube and air vent are clear. Align the air source with the siphon tube. Test with water first.

3. To use, spray inside of piece first in a round and round and up and down pattern. Glaze thickness is deceptive, and thick and thin tend to look alike. Constantly check the glaze thickness with a needle.

4. To dean up, dump glaze back into glaze bucket. Clean gun and tank thoroughly with water, shooting water through the siphon to clear it.

5. Unplug and return to storage locker.

This room must be cleaned after each use! Do not leave glaze materials on any surface. Please report any mess or you will be held responsible.

F‑

GLAZE ERRORS

BLISTERING

Blistering or bubbling apparent in the glaze or on the surface of the glaze. Cause A: Gases escaping from the body during glaze firing. Cure A: Fire the bisque a cone or two higher to make the body less porous. Cause B: Gases escaping from the glaze during firing. Cure B: Dampen the bisque ware. Fire the kiln more slowly and allow for a soaking period of half an hour at the maturing point to allow the broken bubbles to heal over after the gases have escaped. Cause C: Raw alkaline glazes which have sat on the shelf in solution until disassociation gas taken place. Cure C: Grind raw alkaline glazes thoroughly and use them the same day or grind the dry and mix up only the amount needed.

COLOR LOSS

Faded Areas in the glaze fire.

Cause A:                      Overfiring to the point that disassociation of the coloring oxide takes

                 place.

Cure A:                        Watch the kiln temperature more carefully to prevent overfiring.

CRAZING

Cracks formed in the glaze in irregular patterns,

Cause A:                      Differences in the rate of expansion or contraction of the clay body and

                 the glaze, i.e., the glaze shrinks more than the body of the ware.

Cure A:                        Increase the silica content of the clay body by 5% and test the glaze for

                 crazing

Cause B:                      The glaze coating is too thick.

Cure B:                        Apply the glaze in thinner coats.

Cause C:                      Moisture intake in the pottery after firing‑this is known as delayed crazing.

Cure C:                        Use higher fired bisque to retard latent moisture intake.

CRAWLING

Crawling of the glaze into clumps leaving bare areas on the ware. Cause A: Glazing the ware too thickly. Cure A: Use a thinner glaze coat.

Cause B: Overfiring the glaze..

Cure B:                               Fire the glaze one or two cones lower.

Cause C:                        Firing the kiln too rapidly.

Cure C:                          Fire the kiln more slowly.

Cause D:                        Clay content of the glaze too high.

Cure D:                          Decrease the clay content of the glaze recipe by 2%‑3% and test for sign

of crawling.

Cause E:                        Drying the glaze after application too quickly

Cure E:                          Allow the glaze application to dry normally.

Cause F:                       Lack of adhesiveness and tensile strength in the glaze.

Cure F::                        Use a gum adhesive such as C.M.C. in amounts of one teaspoon per

                 pint/half‑liter of glaze.

Cause G:                      Flocculation in storage due to soluble alkalies in the glaze.

Cure G:                        Use glazes with soluble alkalies soon after preparation, or grind and store

                 them dry and mix only the amount needed.

Cause H:                      Greasy or dirty bisque.

Cure H:                        Protect bisque ware from contamination of foreign materials before

                 glazing.

Cause I:                       Overloading the glaze with opacifiers or with feldspar.

Cure 1:                         Decrease the amount of opacifiers or feldspar in the glaze.

Cause J:                       Underfiring.

Cure J:                         Check for other signs of underfiring on the piece and determine if a higher

                 firing is needed.

Cause K:                      Too smooth a body surface, such as burnished ware.

Cure K:                        Do not finish pottery to a very smooth surface if it is to be glazed.

Cause L:                       Double glazing where the first glaze is too dry.

Cure L:                         Apply second glaze while the first glaze is still slightly moist.

Cause M:                      Using water with a very high mineral content

Cure M:                        Use water with a lower mineral content.

DEVITRIFICATION

Areas on the glaze surface which have a frosted crystal‑like appearance.

Cause A:                        Too slow cooling of the kiln.

Cure A:                          Cool the kiln more quickly after the maturing point of the glaze is reached

                  of after the soaking period, if one is employed.

Cause B:                        Too much free silica in the glaze..

Cure B:                          Reduce the amount of free silica content by 5%.

Cause C:                        Too high clay content in the glaze recipe.

Cure C:                          Reduce the clay content by 5%.

EXCESSIVE GLOSS

Excessively high shine on‑the glaze surface. Cause A: Overfiring. Cure A: Fire one cone lower. Cause B: Rapid firing and cooling of the kiln. Cure B: Slow down the firing process. Cause C: Excess flux in the glaze recipe. Cure C: Decrease the fluxing agent by 5%or add kaolin in amounts of 2%‑5% Cause D: Excess silica in the glaze recipe. Cure D: Decrease the silica content in the'glaze formula or add kaolin in 2%‑5% amounts.

GLAZE  RUNNING

Glaze runs down the sides of the ware, pooling at the base and leaving thin areas at

the top.

Cause A:                      Overfiring.

Cure A:                        Watch the kiln more carefully to prevent over‑firing.

Cause B:                      Insufficient kaolin content in the glaze to stabilize it.

Cure B:                        Increase the kaolin content of the glaze by 2%‑5%.

OVERFIRING

Opaque glazes going translucent or clear, matte glazes going shiny, glazes losing their

color, excessive flowing of the glaze, glazes going thin and soaking into the pores of

the ware.

Cause A:                       Overfiring.

Cure A:                         First check to see if the cone level of the glaze has been exceeded, if this

                 is not the case, the glaze may actually have a lower maturing point than

                 indicated in the formula. If the evidence indicates this, fire one cone or

                 two cones lower if necessary.

PINHOLING

Small pinholes in the surface of the glaze.

Cause A:                      Air escaping from a porous under fired bisque ware.

Cure A:                        Fire the bisque one cone higher and dampen the ware before glazing

Cause B:                      Excessive grinding of the glaze.

Cure B:                        Grind the glaze for a shorter period of time.

Cause C:                      Gases escaping from the glaze.

Cure C:                        Allow for a soaking period of half an hour at the maturing point of the glaze

                 to let the gas holes to heal over.

Cause D:                      Glazes which are too viscous

Cure D:                        Increase the flux or decrease the clay content by 2%‑5%

ROUGH SURFACE

Rough, sandy, or grainy surface on the glaze.

Cause A:                       Glaze too thin.

Cure A:                         Apply the glaze more thickly.

Cause B:                       Under firing.

Cure B:                               Fire the glaze ware one or two cones higher.

Cause C:                       Insufficient flux in glaze to melt the silica.

Cure C:                         Increase the fluxing agent by 296‑596

SCUMMING

A frosty scum‑like deposit on the surface of the glaze. Cause A: Soluble salts in the glaze Cure A: Add 2% barium carbonate or a teaspoon of vinegar per pintlhalf‑liter to the glaze mix. Cause B: Sulfur fumes from the fuel where oil or solid fuel is used. Cure B: Use saggers to protect the pots or try another fuel with a lower sulfur content.

SHIVEPPUNG

Glaze breaking away from the body.

Cause A: Takes place when the glaze shrinks more than the glaze, causing a buckling of the glaze.

Cure A: Decrease the silica content of the glaze 5°0‑10%.

SPECKLING

Surface marred by specks of non‑glaze material

Cause A: Foreign material getting into the glaze mix.

Cure A: Protect glaze containers to prevent contamination by foreign materials.

Cause 8: Dust or foreign material getting matter falling on glazed or bisque ware during the drying process.

Cure B: Protect ware during drying stages of both bisque and glazing to prevent foreign matter from settling on them.

UNDERFIRING

Underfired glazes will usually be opaque with rough gritty surfaces and immature

colors as compared to the finished mature glazes.

Cause A: Not firing the glaze to its proper maturing point.

Cure A: Fire the glaze one cone higher and check for signs of underfiring if they are still present, fire the glaze still higher by stages until the proper maturing point of the glaze is determined.