Causes and solutions for running glaze in ceramic mugs
Xin xiang Ceramic Mug Manufacturer, in previous articles, we discussed defects in ceramic mug glazes, including: Glaze Crawling, Orange Peel Glaze, Pinholes and Kiln Dusting. Today, we will talk about another glaze defect: running glaze. running glaze is the opposite of glaze shrinkage. The glaze layer covers areas that it should not cover, forming accumulations. This is running glaze in ceramic mugs.
2.1. Inappropriate glaze formulation: This is the most common cause. The glaze contains too many components that lower the melting point and increase fluidity (such as flux agents: feldspar, borax, etc.), or too few components that increase viscosity and reduce fluidity (such as kaolin, alumina, etc.). It's like adding too much water to chocolate, which naturally makes it runny.
2.2. Glaze layer too thick: When applying glaze, if the glaze layer is too thick on the rim and body of the mug, it will be like a big lump of chocolate. During firing, the thick glaze melts and becomes very heavy, making it more likely to flow downward.
2.3. Firing temperature too high: The actual temperature inside the kiln exceeds the optimal maturation temperature for this glaze formulation. The higher the temperature, the more thoroughly the glaze melts, becoming as thin as water, significantly increasing its flowability and making it more prone to flowing.
2.4. High temperature insulation time is too long: During the ceramic mug firing stage of ceramic mugs, even if the temperature does not exceed the limit, if the glaze remains in a fully molten high-temperature state for an extended period (holding time), it will become increasingly thin and fluid, much like overcooked syrup.
2.5. Improper glaze application: In places where glaze tends to accumulate or where gravity has a significant effect, such as the edges, corners, or base of the handle of a mug, if there is a slight accumulation of glaze, it is more likely to become the starting point for running.
2.6. Shape of the body (unfired mug body, also known as greenware): If the mug is particularly steep or has sharp turns, the glaze is more likely to accumulate and flow downward.
3.1. Glaze droplets or tears: Small bulges or drooping teardrops resembling candle wax form below the rim, at the transition point of the cup body, or at the base of the handle. This is the most typical form of minor glaze flow.
3.2. Uneven accumulation of glaze: The glaze layer at the bottom of the mug is significantly thicker than at the top and may also be darker in color (the thickness of the glaze layer affects the color), making it look like it has “gained weight.”
3.3. Rough and uneven glaze surface: After the flow area cools, the surface may not be smooth and glassy, but may have bumps, orange peel patterns, or unevenness.
3.4. No glaze or thin glaze on the rim of the mug: The glaze has flowed down, causing the glaze on the rim of the mug to become very thin or even expose the white body (commonly known as “de-glazing”), affecting the appearance and usability (drinking feels rough).
3.5. Sticking to the bottom/shelf: In the most serious case, the flowing running glazes directly to the bottom of the mug and sticks to the shelf (usually a refractory material shelf) during the firing process. Forcing them apart when they come out of the kiln will damage the bottom of the mug, rendering the entire mug unusable. The bottom of the mug will be left with unsightly scars or debris from the shelf.
3.6. Contamination of adjacent mugs: If the mugs are placed too close together in the kiln, the flowing glaze may also drip onto the mugs below or next to them, causing defects.
In addition, there are some specially designed areas that are also prone to running glaze.
4.1. Steep mug walls: The mug walls are nearly vertical (with a large angle to the horizontal plane) and lack a curved buffer. Molten running glazes more easily and quickly down steep surfaces, unlike curved mugs, which can “hold” some of the glaze.
4.2. Sharp internal corners/turns: The mug is designed with sharp internal corners and turns (such as certain prismatic mugs or the bottom of decorative grooves). Glaze easily accumulates in these corners and becomes a flow point after melting.
4.3. The rim of the mug is too thin or poorly designed: If the rim of the mug is designed to be very thin and sharp, it will have poor glaze retention. If the glaze is applied a little thickly, it will be more likely to “slip” off the rim and flow down during firing, resulting in a thin glaze or even glaze loss on the rim, as well as glaze flow marks below.
4.4. The connection between the handle and the body of the mug (base): This is also a classic running glaze point. When glazing, the glaze slurry easily accumulates at the base of the handle, and when fired, it flows down from here, forming glaze drips at the base or contaminating the body of the mug.
How can the flowability of glaze at high temperatures be controlled? First, ceramic factories need to optimize the glaze formulation. Increase “thickening agents”: Appropriately increase the content of alumina (Al₂O₃) or kaolin in the formulation to make the molten glaze more viscous, similar to thick yogurt, which is less prone to flowing. Reduce “dilutants”: Appropriately lower the proportion of fluxing agents (such as sodium feldspar, potassium feldspar, borax, calcium carbonate, etc.) to raise the melting point of the glaze and reduce its flowability. Select high-viscosity glazes: Directly choose or develop glaze formulations with inherently higher viscosity at firing temperatures. This requires close communication with glaze suppliers or formulation experiments. Secondly, control the firing temperature. Find the optimal firing temperature: Through multiple test firings (test pieces or small batches of mugs), find the optimal temperature point (glaze maturation temperature range) at which the glaze is mature and its fluidity is controllable, and strictly follow this temperature. Optimize the firing curve: After reaching the maximum firing temperature, avoid excessive heat retention time. Ensure uniform temperature distribution throughout the kiln (minimal temperature differences) to prevent localized overheating that could cause running glaze. Strictly adhere to firing protocols: Establish and strictly follow standardized heating, holding, and cooling curves to ensure consistent firing conditions for each kiln load.
How can localized glaze thickness be avoided? Controlling glaze thickness can be divided into three stages. During glazing, apply the glaze uniformly to avoid excessive thickness: This is the most critical step on the production line! Ensure that workers follow proper procedures, whether it is dipping, spray glazing, or dip glazing, the glaze layer thickness must be uniformly consistent. Pay special attention to areas where glaze tends to accumulate, such as the rim, corners, and base of the handle, and do not apply too thick a layer. Use a glaze hydrometer and viscometer to monitor the stability of the glaze slurry. After glazing is complete, before placing the pieces in the kiln, check manually or with machine vision to see if there are any areas where the glaze is too thick, and treat them in a timely manner (for example, gently remove excess glaze with a sponge or brush). When placing the cups in the kiln, apply a thin layer of aluminum oxide powder or a special refractory coating (commonly known as “kiln sand” or “anti-adhesive sand”) to the shelf to prevent the cups from sticking to the shelf, even if a small amount of glaze flows to the bottom, thereby reducing losses.
1. What is running glaze?
running glaze occurs when ceramic glaze, due to excessive fluidity or prolonged exposure to high temperatures, flows downward like melted syrup, exceeding the originally designed area. The rim, body, or handle of the mug, which should be smooth and flat, will form irregular glaze accumulations and drip marks, and may even flow to the bottom of the mug and stick to the shelf (the rack that supports the mug during firing), causing defects or waste. For example, when we make chocolate cake, we pour melted chocolate evenly over the cake. If not controlled properly, the chocolate will flow too much or too much chocolate will be poured, causing chocolate to accumulate at the bottom of the cake. If we consider melted chocolate as glaze, then this situation is similar to running glaze in ceramic mugs.2. Causes of running glaze
The occurrence of glaze flow stems from the glaze becoming too fluid or remaining in a fluid state for too long when it melts into a “glass liquid” state in the kiln. The main causes include the following:2.1. Inappropriate glaze formulation: This is the most common cause. The glaze contains too many components that lower the melting point and increase fluidity (such as flux agents: feldspar, borax, etc.), or too few components that increase viscosity and reduce fluidity (such as kaolin, alumina, etc.). It's like adding too much water to chocolate, which naturally makes it runny.
2.2. Glaze layer too thick: When applying glaze, if the glaze layer is too thick on the rim and body of the mug, it will be like a big lump of chocolate. During firing, the thick glaze melts and becomes very heavy, making it more likely to flow downward.
2.3. Firing temperature too high: The actual temperature inside the kiln exceeds the optimal maturation temperature for this glaze formulation. The higher the temperature, the more thoroughly the glaze melts, becoming as thin as water, significantly increasing its flowability and making it more prone to flowing.
2.4. High temperature insulation time is too long: During the ceramic mug firing stage of ceramic mugs, even if the temperature does not exceed the limit, if the glaze remains in a fully molten high-temperature state for an extended period (holding time), it will become increasingly thin and fluid, much like overcooked syrup.
2.5. Improper glaze application: In places where glaze tends to accumulate or where gravity has a significant effect, such as the edges, corners, or base of the handle of a mug, if there is a slight accumulation of glaze, it is more likely to become the starting point for running.
2.6. Shape of the body (unfired mug body, also known as greenware): If the mug is particularly steep or has sharp turns, the glaze is more likely to accumulate and flow downward.
3. Forms of running glaze
The appearance of running glaze varies depending on its severity and location. Common forms include:3.1. Glaze droplets or tears: Small bulges or drooping teardrops resembling candle wax form below the rim, at the transition point of the cup body, or at the base of the handle. This is the most typical form of minor glaze flow.
3.2. Uneven accumulation of glaze: The glaze layer at the bottom of the mug is significantly thicker than at the top and may also be darker in color (the thickness of the glaze layer affects the color), making it look like it has “gained weight.”
3.3. Rough and uneven glaze surface: After the flow area cools, the surface may not be smooth and glassy, but may have bumps, orange peel patterns, or unevenness.
3.4. No glaze or thin glaze on the rim of the mug: The glaze has flowed down, causing the glaze on the rim of the mug to become very thin or even expose the white body (commonly known as “de-glazing”), affecting the appearance and usability (drinking feels rough).
3.5. Sticking to the bottom/shelf: In the most serious case, the flowing running glazes directly to the bottom of the mug and sticks to the shelf (usually a refractory material shelf) during the firing process. Forcing them apart when they come out of the kiln will damage the bottom of the mug, rendering the entire mug unusable. The bottom of the mug will be left with unsightly scars or debris from the shelf.
3.6. Contamination of adjacent mugs: If the mugs are placed too close together in the kiln, the flowing glaze may also drip onto the mugs below or next to them, causing defects.
4. Which shapes of mugs are more prone to running glaze?
Many ceramic mug buyers sometimes find that some custom ceramic mugs are prone to running glaze, even when the manufacturing plant is changed. In fact, the reason why this ceramic mug has running glaze is related to its design. More specifically, this ceramic mug is more prone to running glaze. So, what do ceramic mugs that are more prone to running glaze have in common? When customizing ceramic mugs, can we reduce this design? The answer is yes. There is a type of ceramic mug with a raised bottom and a straight body. Inside the mug, there is a deep groove where the bottom meets the body. This area is prone to running glaze. When glazing this type of mug (especially with an internal glaze, which is often done using the dipping method), the glaze will naturally flow to the lowest point—that is, the “ring-shaped groove” or “ring-shaped valley” area formed around the highest point of the raised bottom (i.e., the junction between the cup wall and the convex bottom). This location is like a natural reservoir, accumulating a glaze layer that is much thicker than the cup wall or the top of the protrusion.In addition, there are some specially designed areas that are also prone to running glaze.
4.1. Steep mug walls: The mug walls are nearly vertical (with a large angle to the horizontal plane) and lack a curved buffer. Molten running glazes more easily and quickly down steep surfaces, unlike curved mugs, which can “hold” some of the glaze.
4.2. Sharp internal corners/turns: The mug is designed with sharp internal corners and turns (such as certain prismatic mugs or the bottom of decorative grooves). Glaze easily accumulates in these corners and becomes a flow point after melting.
4.3. The rim of the mug is too thin or poorly designed: If the rim of the mug is designed to be very thin and sharp, it will have poor glaze retention. If the glaze is applied a little thickly, it will be more likely to “slip” off the rim and flow down during firing, resulting in a thin glaze or even glaze loss on the rim, as well as glaze flow marks below.
4.4. The connection between the handle and the body of the mug (base): This is also a classic running glaze point. When glazing, the glaze slurry easily accumulates at the base of the handle, and when fired, it flows down from here, forming glaze drips at the base or contaminating the body of the mug.
5. Which colors of mugs are more prone to running glaze?
Have you noticed that many of the ceramic mugs with running glaze are dark in color? White ceramic mugs rarely have running glaze. Could it be that color also has something to do with running glaze? The answer is yes. Dark-colored ceramic mugs, such as black, brown, dark red, and green ceramic mugs, are prone to running glaze. If the mug has a special design, the chance of running glaze is even greater. So, what exactly is the reason for this? In fact, it all comes down to the composition of the glaze. Dark-colored glazes derive their color from the addition of metal oxides (such as iron oxide Fe₂O₃, manganese oxide MnO₂, chromium oxide Cr₂O₃, cobalt oxide CoO, etc.). These oxides not only determine the glaze's color but also act as strong fluxing agents, enhancing the glaze's fluidity. During firing, this increased fluidity makes running glaze more likely.6. Methods to Reduce running glaze
running glaze is a common issue encountered by ceramic factories during the production of mugs. To reduce running glaze and address this issue, there are two key points: first, controlling the flowability of the glaze at high temperatures, and second, avoiding localized thickening of the glaze layer.How can the flowability of glaze at high temperatures be controlled? First, ceramic factories need to optimize the glaze formulation. Increase “thickening agents”: Appropriately increase the content of alumina (Al₂O₃) or kaolin in the formulation to make the molten glaze more viscous, similar to thick yogurt, which is less prone to flowing. Reduce “dilutants”: Appropriately lower the proportion of fluxing agents (such as sodium feldspar, potassium feldspar, borax, calcium carbonate, etc.) to raise the melting point of the glaze and reduce its flowability. Select high-viscosity glazes: Directly choose or develop glaze formulations with inherently higher viscosity at firing temperatures. This requires close communication with glaze suppliers or formulation experiments. Secondly, control the firing temperature. Find the optimal firing temperature: Through multiple test firings (test pieces or small batches of mugs), find the optimal temperature point (glaze maturation temperature range) at which the glaze is mature and its fluidity is controllable, and strictly follow this temperature. Optimize the firing curve: After reaching the maximum firing temperature, avoid excessive heat retention time. Ensure uniform temperature distribution throughout the kiln (minimal temperature differences) to prevent localized overheating that could cause running glaze. Strictly adhere to firing protocols: Establish and strictly follow standardized heating, holding, and cooling curves to ensure consistent firing conditions for each kiln load.
How can localized glaze thickness be avoided? Controlling glaze thickness can be divided into three stages. During glazing, apply the glaze uniformly to avoid excessive thickness: This is the most critical step on the production line! Ensure that workers follow proper procedures, whether it is dipping, spray glazing, or dip glazing, the glaze layer thickness must be uniformly consistent. Pay special attention to areas where glaze tends to accumulate, such as the rim, corners, and base of the handle, and do not apply too thick a layer. Use a glaze hydrometer and viscometer to monitor the stability of the glaze slurry. After glazing is complete, before placing the pieces in the kiln, check manually or with machine vision to see if there are any areas where the glaze is too thick, and treat them in a timely manner (for example, gently remove excess glaze with a sponge or brush). When placing the cups in the kiln, apply a thin layer of aluminum oxide powder or a special refractory coating (commonly known as “kiln sand” or “anti-adhesive sand”) to the shelf to prevent the cups from sticking to the shelf, even if a small amount of glaze flows to the bottom, thereby reducing losses.
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