What is the effect of fluxes in glaze? Allow your students to predict the final product of glazes with their chemistry knowledge.
In this exercise, your students will create their own small batch clear glazes to observe the effect of calcium carbonate as a flux. Students will take an experimental approach, creating multiple batches with varying amounts of flux while holding other materials constant.
Learning Objectives
Students are able to describe how 3D silica structure impacts glaze formation and firing outcomes from the kiln. Students should be able to describe what the constituent elements of a glaze recipe represent, and what changing the ratios of these constituents will produce in a final fired piece. Students can explain what defects occur from high firing glazes, and the chemical background from which these arise.
Required Materials
A full studio is recommended for this exercise. Specific equipment required include:
High fire kiln (up to 2500°C)
Recommended recipe for clear glaze (adapted from Orenstein, 1989)
71g silica
54.4g kaolin (source of alumina)
74.6g calcium carbonate (flux)
200-400mL water
Porcelain test tiles (3.5” x 1” standard, students may use other pieces if they are consistent across dimensions)
NOTE: For glaze recipe, make sure you have enough materials for five batches per student. Each student must create an array of glaze recipes, which is material and space intensive- plan accordingly.
In particular, make sure you have ~100g extra of calcium carbonate per student per glaze, as this will be the ingredient varied across recipes.
Directions
Step 1 - Mix
Students will begin by creating a stock batch of a clear glazer recipe, with approx. 35:27:38 silica to alumina to flux ratio with equal volume water (200mL example given in required materials). Kaolin and calcium carbonate are recommended for the alumina and flux sources. Students must produce enough glaze to have a total of 500mL total, it is recommended that they prepare these batches separately to ensure little mistakes are made.
Using the ratio given above up, add dry ingredients first, then add water until powders are fully emulsified. From the given recipe, add an additional 10g of calcium carbonate into each batch, until you reach 50g. For example, your first batch will contain 48g, and the last 88g. Ensure the separate batches are labeled and kept apart. Each student should have a total of five batches of glaze, with a minimum volume of 100mL each by the end of this step.
Constraints: Silica can be incredibly toxic if inhaled- make sure your students are provided N95 masks in a well ventilated area before beginning, and ensure all work stations are properly cleaned before finishing.
This exercise is incredibly resource taxing, with five batches required per student. At the end, you may be able to combine all glazes into one batch- this batch will need some additional silica and kaolin to adjust for the added flux, however a functional clear glaze should be salvageable- recycle your materials!
Step 2 - Glaze
With the five batches now mixed, provide your students test tiles with which to glaze pieces. Depending on class size, students can do more than one tile per batch, however just five is sufficient for the desired result from this exercise. Students should dip each tile approx. 1.5 inches from the bottom, leaving room for the glaze to run down.
Key Observations: Students should observe the relative consistencies between glazes- glazes with higher proportions of calcium carbonate will have be more grainy, and clump to the pieces more than the batches with the minimal amount.
Step 3 - Fire
Fire all pieces made by students up to cone 10, with pieces made by each student being fired together to reduce variability when comparing glaze formulations. Ensure the students are able to track which pieces correspond to which batch was made, and to which student each belongs. Organize the pieces from lowest to highest concentration of flux, and give them to students.
Measure: Students should attempt to measure the amount of glaze which has run down the piece. Using their constant point of glazing (1.5" from bottom of test tile), they can measure how much further down the piece each glaze has run. These measurements are to be compared qualitatively- it is not necessary for the measurements to be exactly correct.
Constraints: It is preferable that kiln loads for firing be kept low- the larger a volume of pieces within one firing gets, the more varied disbursement of heat is throughout the kiln. This causes the temperature around a piece to fluctuate, diminishing any impact the flux may have in melting the glaze.
Step 4 - Analyze
Students should now have their test tiles fully fired and organized. Many of the pieces with high flux may be cracked or broken from the firing- this is not a mistake but an intended outcome from the high levels of flux. Again have students compare their pieces with each other, noticing general trends rather than exact issues with their own batch.
Key Observations: Students should see two main differences between the low and high flux recipes: the glossiness of the finish, and how far down the piece the glaze has run. In the lower flux amounts, the glaze will be matte and barely run down the piece, as the silica mixture in the glaze requires a higher temperature to melt, having less time to run down. Higher flux amounts lower the melting temperature, allowing the glaze more time in the kiln to melt down the piece.
Constraints: Batch variability will be high across students, but pieces made by the same student should be able to highlight the significant trends. Without the addition of coloration, the quality of matte or glossiness may be difficult to see. This is a minor issue, as the distance run down the pot is the observation important to intuiting the role of a flux in a glaze.
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